CEPHALON INC
8-K, EX-20.1, 2000-10-18
PHARMACEUTICAL PREPARATIONS
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                                                                    Exhibit 20.1

                                 [ANESTA LOGO]

                     SPECIAL MEETING OF ANESTA STOCKHOLDERS

                 MERGER PROPOSED -- YOUR VOTE IS VERY IMPORTANT

     The board of directors of Anesta Corp. has unanimously approved a merger
with Cephalon, Inc.

     We cannot complete the merger unless the holders of Anesta common stock
approve the Agreement and Plan of Merger, dated as of July 14, 2000, among
Cephalon, C Merger Sub, Inc., a wholly-owned subsidiary of Cephalon, and Anesta.
Under the merger agreement, C Merger Sub will be merged with and into Anesta and
Anesta will become a wholly-owned subsidiary of Cephalon.

     If the merger is completed, holders of Anesta common stock will receive
0.4765 of a share of Cephalon common stock for each share of Anesta common stock
they own. Cash will be paid in lieu of fractional shares. This is a fixed
exchange ratio that will not be adjusted for changes in the stock price of
either company before the merger is completed. The Cephalon common stock is
listed on the Nasdaq National Market under the symbol "CEPH." The Anesta common
stock is listed on the Nasdaq National Market under the symbol "NSTA."

     Anesta has scheduled a special meeting of its stockholders to consider and
vote upon a proposal to approve and adopt the merger agreement. The date, time
and place of the special meeting are as follows:

                                October 10, 2000
                               10:00 a.m., M.D.T.
                                  Anesta Corp.
                              4745 Wiley Post Way
                           Salt Lake City, Utah 84116

     This proxy statement/prospectus provides you with information about
Cephalon, Anesta and the proposed merger. In addition, you may obtain other
information about Cephalon and Anesta from documents filed with the Securities
and Exchange Commission. We encourage you to read the entire merger agreement
and this proxy statement/prospectus carefully, AND WE ESPECIALLY ENCOURAGE YOU
TO READ THE SECTION ON "RISK FACTORS" BEGINNING ON PAGE 13.

     YOUR VOTE IS IMPORTANT. Whether or not you plan to attend the special
meeting, please take the time to vote by completing and mailing the enclosed
proxy card to us. If you sign, date and mail your proxy card without indicating
how you wish to vote, your proxy will be counted as a vote in favor of the
proposal to approve the merger agreement. If you fail to return your proxy card,
the effect will be a vote against the proposal to approve the merger agreement.

                                            [Stanley Signature]
                                            Theodore H. Stanley, M.D.
                                            Secretary

     NEITHER THE SECURITIES AND EXCHANGE COMMISSION NOR ANY STATE SECURITIES
COMMISSION HAS APPROVED OR DISAPPROVED OF THE CEPHALON COMMON STOCK TO BE ISSUED
IN THE MERGER OR PASSED UPON THE ADEQUACY OR ACCURACY OF THIS PROXY
STATEMENT/PROSPECTUS. ANY REPRESENTATION TO THE CONTRARY IS A CRIMINAL OFFENSE.

     This proxy statement/prospectus is dated September 8, 2000, and is first
being mailed to stockholders of Anesta on or about September 11, 2000.
<PAGE>   2

     This proxy statement/prospectus constitutes the proxy statement of Anesta
in connection with the solicitation of proxies by the Anesta board of directors
for use at the special meeting of Anesta stockholders and at any adjournments or
postponements of the special meeting. This proxy statement/prospectus also
constitutes the prospectus of Cephalon for use in connection with the offer and
issuance of shares of Cephalon common stock, $0.01 par value per share, to
Anesta stockholders pursuant to the merger.

     This proxy statement/prospectus incorporates important business and
financial information about Cephalon and Anesta that is not included in or
delivered with this document. This information is available without charge to
Anesta stockholders upon written or oral request. Stockholders should contact:
CEPHALON, INC., 145 Brandywine Parkway, West Chester, Pennsylvania 19380,
Attention: Investor Relations, Phone Number: (610) 344-0200 or ANESTA CORP.,
4745 Wiley Post Way, Salt Lake City, Utah 84116, Attention: Investor Relations,
Phone Number: (801) 595-1405.

     TO OBTAIN TIMELY DELIVERY OF REQUESTED DOCUMENTS BEFORE THE SPECIAL MEETING
OF ANESTA STOCKHOLDERS, YOU MUST REQUEST THEM NO LATER THAN OCTOBER 3, 2000,
WHICH IS FIVE BUSINESS DAYS BEFORE THE DATE OF THE SPECIAL MEETING OF ANESTA
STOCKHOLDERS.

     Also see "Where You Can Find More Information" in this proxy
statement/prospectus beginning on page 58.
<PAGE>   3

                                  ANESTA CORP.
                              4745 WILEY POST WAY
                           SALT LAKE CITY, UTAH 84116
                                 (801) 595-1405

                   NOTICE OF SPECIAL MEETING OF STOCKHOLDERS
                         TO BE HELD ON OCTOBER 10, 2000

TO THE STOCKHOLDERS OF ANESTA CORP.:

     NOTICE IS HEREBY GIVEN that a special meeting of stockholders of Anesta
Corp., a Delaware corporation, will be held on October 10, 2000 at 10:00 a.m.,
local time, at the principal executive offices of Anesta located at 4745 Wiley
Post Way, Salt Lake City, Utah 84116, for the following purposes:

     - To consider and vote on a proposal to approve and adopt the Agreement and
       Plan of Merger, dated as of July 14, 2000, among Anesta, Cephalon, Inc.,
       and C Merger Sub, Inc., a wholly-owned subsidiary of Cephalon.

     - To transact such other business as may be properly brought before the
       special meeting and any adjournments or postponements thereof.

     The proposed merger and other related matters are more fully described in
the attached proxy statement/ prospectus.

     The Anesta board of directors has fixed the close of business on September
5, 2000 as the record date for the determination of stockholders entitled to
notice of, and to vote at, the special meeting and any adjournments or
postponements thereof. At the close of business on the record date, Anesta had
outstanding and entitled to vote 13,451,716 shares of common stock at the
special meeting.

     THE ANESTA BOARD OF DIRECTORS HAS DETERMINED THAT THE MERGER AGREEMENT AND
THE TRANSACTIONS CONTEMPLATED BY IT ARE IN THE BEST INTERESTS OF ANESTA AND ITS
STOCKHOLDERS. ACCORDINGLY, THE ANESTA BOARD OF DIRECTORS UNANIMOUSLY RECOMMENDS
THAT ANESTA STOCKHOLDERS VOTE TO APPROVE THE MERGER AGREEMENT.

     Even if you plan to attend the special meeting in person, we request that
you sign and return the enclosed proxy to ensure that your shares will be
represented at the special meeting if you are unable to attend. If you sign,
date and mail your proxy card without indicating how you wish to vote, your
proxy will be counted as a vote in favor of the proposal to approve the merger
agreement. If you fail to return your proxy card, the effect will be a vote
against the proposal to approve the merger agreement. If you do attend the
special meeting and wish to vote in person, you may withdraw your proxy by
delivering a written notice dated later than the date on your proxy card to the
secretary of Anesta and then vote in person.

     YOUR VOTE IS IMPORTANT. THE AFFIRMATIVE VOTE OF THE HOLDERS OF A MAJORITY
OF THE OUTSTANDING SHARES OF ANESTA COMMON STOCK IS REQUIRED TO APPROVE THE
MERGER AGREEMENT.

                                            By Order of the Board of Directors,

                                            [Stanley Signature]
                                            Theodore H. Stanley, M.D.
                                            Secretary

Salt Lake City, Utah
September 8, 2000
<PAGE>   4

                               TABLE OF CONTENTS

<TABLE>
<CAPTION>
                                                              PAGE
                                                              ----
<S>                                                           <C>
QUESTIONS AND ANSWERS ABOUT THE MERGER......................    1
SUMMARY.....................................................    3
SELECTED CONSOLIDATED FINANCIAL DATA........................    8
  Cephalon Selected Consolidated Financial Data.............    8
  Anesta Selected Financial Data............................    9
SUMMARY UNAUDITED PRO FORMA COMBINED FINANCIAL DATA.........   10
SUMMARY UNAUDITED COMPARATIVE PER SHARE DATA................   12
RISK FACTORS................................................   13
FORWARD LOOKING INFORMATION.................................   14
THE SPECIAL MEETING OF ANESTA STOCKHOLDERS..................   15
  Date, Time and Place......................................   15
  Purpose of the Special Meeting............................   15
  Recommendation of the Anesta Board of Directors...........   15
  Record Date and Voting Power..............................   15
  Voting and Revocation of Proxies..........................   15
  Quorum; Required Vote.....................................   15
  Solicitation of Proxies...................................   16
  No Appraisal Rights.......................................   16
  Other Matters.............................................   16
THE MERGER..................................................   17
  General Description of the Merger.........................   17
  Background................................................   17
  Reasons for the Merger....................................   20
  Opinion of Anesta's Financial Advisor.....................   22
  Interests of Anesta's Officers and Directors in the
     Merger.................................................   29
  Material Federal Income Tax Consequences..................   31
  Anticipated Accounting Treatment..........................   33
  Regulatory Approvals......................................   33
  Restrictions on Resale by Affiliates......................   34
CERTAIN TERMS OF THE MERGER AGREEMENT.......................   34
  General...................................................   34
  Closing Matters...........................................   34
  Conversion of Shares; Treatment of Stock Options..........   35
  Exchange of Stock Certificates............................   35
  Representations and Warranties............................   35
  Covenants.................................................   37
  Conditions to Obligations to Effect the Merger............   39
  Termination; Termination Fees and Expenses................   40
  Amendment and Waiver......................................   41
THE COMPANIES...............................................   42
  Cephalon..................................................   42
  C Merger Sub..............................................   42
  Anesta....................................................   42
UNAUDITED PRO FORMA FINANCIAL INFORMATION...................   44
MARKET PRICE DATA AND DIVIDEND POLICIES.....................   53
MANAGEMENT AND OTHER INFORMATION............................   54
</TABLE>

                                        i
<PAGE>   5

<TABLE>
<CAPTION>
                                                              PAGE
                                                              ----
<S>                                                           <C>
COMPARISON OF THE RIGHTS OF THE HOLDERS OF CEPHALON COMMON
  STOCK AND ANESTA COMMON STOCK.............................   54
  Stockholder Action by Consent.............................   54
  Special Stockholders' Meetings............................   54
  Advance Notice of Director Nominations and Other
     Stockholder Proposals..................................   55
  Number and Election of Directors..........................   55
  Amendment of Bylaws.......................................   55
  Removal of Directors......................................   56
  Vacancies.................................................   56
  Preferred Stock...........................................   56
  Rights Plan...............................................   57
INDEPENDENT ACCOUNTANTS.....................................   57
LEGAL MATTERS...............................................   57
EXPERTS.....................................................   57
WHERE YOU CAN FIND MORE INFORMATION.........................   58
  Cephalon Filings..........................................   58
  Anesta Filings............................................   58
ANNEX A -- Agreement and Plan of Merger.....................  A-1
ANNEX B -- U.S. Bancorp Piper Jaffray Opinion...............  B-1
</TABLE>

                                       ii
<PAGE>   6


                     QUESTIONS AND ANSWERS ABOUT THE MERGER

Q:    WHY DOES ANESTA WANT TO MERGE WITH CEPHALON?

A.    The merger will provide a strategic and complementary fit with both the
      existing and envisioned future businesses of Cephalon and Anesta.

      For more detailed reasons for the merger, see pages 20 through 22.

Q:    DOES THE ANESTA BOARD OF DIRECTORS RECOMMEND APPROVAL OF THE MERGER
      AGREEMENT?

A.    Yes. After careful consideration, the Anesta board of directors
      unanimously recommends that the stockholders vote in favor of the merger
      agreement and the merger. Additional information pertaining to the
      recommendation of the board of directors can be found under "The Merger --
      Reasons for the Merger."

Q:    WHAT WILL I RECEIVE IN THE MERGER?

A.    The merger will result in the exchange of outstanding shares of Anesta
      common stock in the ratio of 0.4765 of a share of Cephalon common stock
      for each share of Anesta common stock. You will not receive any fractional
      share. Instead, you will receive cash in an amount equal to the average
      closing price for a share of Cephalon common stock for the ten day trading
      period ending two days before the merger multiplied by the appropriate
      fraction.

Q:    WILL CEPHALON STOCKHOLDERS RECEIVE ANY SHARES AS A RESULT OF THE MERGER?

A.    No. Cephalon stockholders will continue to hold the Cephalon shares they
      own at the time of the merger.

Q:    WHAT WILL HAPPEN TO ANESTA AS A RESULT OF THE MERGER?

A.    If the merger is completed, Anesta will become a wholly-owned subsidiary
      of Cephalon, and the Anesta common stock will cease to be traded on the
      Nasdaq National Market.

Q:    WHAT HAPPENS IF THE MERGER IS NOT COMPLETED?

A.    If the merger is not completed, both companies will continue to operate as
      independent companies. Anesta may be required to pay a termination fee and
      certain expenses to Cephalon under the merger agreement if the merger is
      not completed for certain reasons. Cephalon will also be required to pay
      certain expenses to Anesta if the merger is not completed for certain
      reasons.

Q:    WHERE CAN I GET INFORMATION REGARDING CEPHALON, ANESTA AND THE MERGER?

A.    We urge you to read and consider the information contained in this proxy
      statement/prospectus, including its appendices. You should also review the
      documents referenced under "Where You Can Find More Information."

Q:    WHO MAY VOTE AT THE SPECIAL MEETING?

A.    All Anesta stockholders of record as of the close of business on
      September 5, 2000 may vote at the special meeting. You are entitled to one
      vote per share of Anesta common stock that you own on the record date.

Q:    WHAT AM I BEING ASKED TO VOTE UPON IN CONNECTION WITH THE MERGER?

A.    You are being asked to vote for the approval
      and adoption of the merger agreement.

Q:    WHAT DO I NEED TO DO NOW?

A.    After reviewing this document, indicate on your proxy card how you want
      to vote, sign it and mail it in the enclosed postage prepaid return
      envelope as soon as possible so that the proxyholder may vote your shares
      at the special meeting.

Q:    WHEN IS THE SPECIAL MEETING?

A.    The special meeting will take place on October 10, 2000.

Q:    HOW WILL MY SHARES BE VOTED IF I RETURN A BLANK PROXY CARD?

A.    If you sign and send in your proxy card and do not indicate how you want
      to vote, we will count your proxy as a vote in favor of the proposals
      submitted at the special meeting.

Q:    WHAT WILL BE THE EFFECT IF I DO NOT VOTE ON THE MERGER PROPOSAL?

A.    If you abstain from voting or do not vote your shares by proxy or in
      person, it will have the same effect as a vote against adoption of the
      merger agreement.

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<PAGE>   7

Q:    CAN I VOTE MY SHARES IN PERSON?

A.    If you hold your shares as the record holder and not in "street name,"
      you may attend the special meeting and vote your shares in person, rather
      than signing and mailing your proxy card.

Q:    IF MY SHARES ARE HELD IN "STREET NAME" BY MY BROKER, WILL MY BROKER VOTE
      MY SHARES FOR ME?

A.    Your broker will vote your shares on the merger proposal only if you
      instruct your broker how to vote. Your broker will send you directions on
      how you can instruct your broker to vote. If you do not instruct your
      broker, your shares will not be voted, which will have the same effect as
      a vote against the adoption of the merger agreement.

Q:    CAN I REVOKE MY PROXY AND CHANGE MY VOTE?

A.    Yes. If you hold your shares as the record holder, you may change your
      vote in one of three ways at any time before your proxy is voted at the
      special meeting. First, you may send a written notice stating that you
      would like to revoke your proxy. Second, you may complete and submit a
      new, later dated proxy. Third, you may attend the special meeting and vote
      in person. If you choose either of the first two methods, you must submit
      your notice of revocation or your new proxy to the Secretary of Anesta.

      If you hold your shares in "street name" and have instructed your broker
      to vote your shares, you must follow directions received from your broker
      to change those instructions.

Q:    SHOULD I SEND IN MY STOCK CERTIFICATES NOW?

A.    No. After we complete the merger, Cephalon will send you written
      instructions on how to exchange your stock certificates.

Q:    WHEN DO YOU EXPECT TO COMPLETE THE MERGER?

A.    We currently expect to complete the merger during the fourth quarter of
      2000 if we obtain the required stockholder approval at the special meeting
      and satisfy certain additional conditions. However, subject to certain
      exceptions, either Anesta or Cephalon can terminate the merger agreement
      if the merger is not completed by December 31, 2000.

Q:    WHAT ARE THE TAX CONSEQUENCES OF THE MERGER?

A.    The exchange of shares in the merger will be tax-free to you for U.S.
      federal income tax purposes, except for taxes payable on any gain
      recognized as a result of receiving cash in lieu of a fractional share of
      Cephalon common stock. A summary of the material federal income tax
      consequences of the merger is included in the section "The
      Merger--Material Federal Income Tax Consequences" on page 31.

Q:    AM I ENTITLED TO APPRAISAL RIGHTS IN CONNECTION WITH THE MERGER?

A.    No. You do not have appraisal rights in connection with the merger.

Q:    WHO CAN HELP ANSWER MY QUESTIONS?

A.    If you have more questions about the merger or if you need additional
      copies of this proxy statement/prospectus or the enclosed proxy, you
      should contact:

          Anesta Corp.
          4745 Wiley Post Way
          Salt Lake City, UT 84116
          Attention: Roger Evans
          Telephone Number: (801) 595-1405

                                        2
<PAGE>   8

                                    SUMMARY

     This summary highlights selected information from this document and may not
contain all the information that is important to you. To understand the merger
fully, you should read carefully this entire document and the documents to which
we refer. See "Where You Can Find More Information" on page 58. The merger
agreement is attached as Annex A to this proxy statement/prospectus. We
encourage you to read the merger agreement as it is the legal document that
governs the merger. We have included page references in parentheses to direct
you to a more detailed description of the topics presented in this summary.

THE COMPANIES (PAGE 42)

  Cephalon, Inc.
  145 Brandywine Parkway, West Chester, Pennsylvania 19380 (610) 344-0200

     Cephalon, Inc., with its principal offices located in West Chester,
Pennsylvania, is a biopharmaceutical company dedicated to the discovery,
development and marketing of products to treat neurological disorders and
cancer. Cephalon markets Provigil(R) (modafinil) Tablets [C-IV] for treating
excessive daytime sleepiness associated with narcolepsy and is conducting
clinical studies concerning the use of Provigil in other areas. Cephalon's other
research programs seek to discover and develop treatments for neurological
disorders and cancer, including Parkinson's disease, Alzheimer's disease and
prostate cancer. Cephalon also enters into collaborative commercial arrangements
under which it markets the products of third parties, including an arrangement
with Abbott Laboratories to market Gabitril(R) for the treatment of partial
seizures associated with epilepsy. Cephalon was incorporated in Delaware in
1987.

  C Merger Sub, Inc.
  145 Brandywine Parkway, West Chester, Pennsylvania 19380 (610) 344-0200

     C Merger Sub is a wholly-owned subsidiary of Cephalon formed in Delaware
for the purpose of effecting the merger.

  Anesta Corp.
  4745 Wiley Post Way, Salt Lake City, Utah 84116 (801) 595-1405

     Anesta is a leader in the development of new pharmaceutical products using
its proprietary oral transmucosal system (OTS(TM)) for drug delivery. Anesta was
incorporated in 1985 under the laws of Utah to commercialize specific licensed
technologies from the University of Utah, and reincorporated in 1993 under the
laws of Delaware. Anesta's lead product is Actiq(R) (oral transmucosal fentanyl
citrate), which was approved by the U.S. Food and Drug Administration, or FDA,
for marketing in November 1998 and launched in the U.S. in March 1999. Actiq is
indicated only for the management of breakthrough cancer pain in patients with
malignancies who are already receiving and who are tolerant to opioid therapy
for their underlying persistent cancer pain.

THE MERGER (PAGE 17)

     Under the merger agreement, a wholly-owned subsidiary of Cephalon will
merge with and into Anesta. After this merger, Anesta will be a wholly-owned
subsidiary of Cephalon and Anesta stockholders will become Cephalon
stockholders. Stockholders of Anesta will receive common stock of Cephalon in
the merger in exchange for their Anesta common stock.

MERGER CONSIDERATION; FIXED EXCHANGE RATIO (PAGE 35)

     In the merger, you will receive 0.4765 of a share of Cephalon common stock
in exchange for each share of Anesta common stock you own. The actual number of
shares you will receive in the merger will be 0.4765 multiplied by the number of
shares of Anesta common stock that you own at the effective time of the merger,
except that you will receive cash for any fractional share. For example, if you
own 100 shares of

                                        3
<PAGE>   9

Anesta common stock, you will receive 47 shares of Cephalon common stock, and a
check for the market value of the 0.65 fractional share.

     This exchange ratio is fixed. Regardless of fluctuations in the market
prices of Cephalon's or Anesta's common stock, the exchange ratio will not
change between now and the date that the merger is completed.

REASONS FOR THE MERGER (PAGE 20)

     Cephalon (Page 20). The Cephalon board of directors approved the merger
based on a number of factors, including the following:

     - the belief that through the acquisition of Anesta's Actiq marketing
       rights, Cephalon will shorten its path to profitability;

     - the enhanced product development opportunities for Cephalon through the
       addition of Anesta's OTS system for drug delivery; and

     - the potential to realize synergies in research and development,
       technology and commercialization and through the elimination of
       redundancies.

     Anesta (Page 21). The Anesta board of directors believes that the merger
could result in a number of benefits to Anesta and its stockholders, including,
among other benefits, the following:

     - the creation of a combined company with greater resources, a more
       diversified product portfolio and greater management, sales, services and
       marketing capabilities;

     - the potential to expand the market presence of Actiq and leverage the
       marketing of the combined companies products to some of the same patient
       profiles;

     - the belief that the combined company would have better access to capital;
       and

     - the advantages of combining Anesta's manufacturing capabilities with
       Cephalon's products.

RECOMMENDATION TO ANESTA STOCKHOLDERS

     The Anesta board of directors believes that the merger is advisable and
unanimously recommends that you vote "FOR" approval of the merger agreement.

OPINION OF ANESTA'S FINANCIAL ADVISOR (PAGE 22)

     In deciding to approve the merger, one of the factors that the Anesta board
of directors considered was the opinion of its financial advisor, U.S. Bancorp
Piper Jaffray, Inc., that, as of July 14, 2000 and subject to the considerations
set forth in the opinion, the exchange ratio pursuant to the merger agreement
was fair, from a financial point of view, to the holders of Anesta common stock.
U.S. Bancorp Piper Jaffray's opinion was provided for the information and
assistance of the Anesta board of directors in connection with its consideration
of the transaction contemplated by the merger agreement and does not constitute
a recommendation as to how any holder of Anesta common stock should vote with
respect to the merger agreement. The full text of the U.S. Bancorp Piper Jaffray
opinion (which sets forth assumptions made, matters considered and limitations
on the review undertaken by U.S. Bancorp Piper Jaffray in connection with the
opinion) is attached as Annex B to this proxy statement/prospectus. WE URGE YOU
TO READ THE ENTIRE OPINION CAREFULLY.

INTERESTS OF ANESTA OFFICERS AND DIRECTORS IN THE MERGER (PAGE 29)

     When considering the recommendation by the Anesta board of directors, you
should be aware that a number of Anesta's officers and directors have interests
in the merger that are different from other Anesta stockholders. The Anesta
board of directors took into account these interests prior to making its
decision to approve the merger agreement.

                                        4
<PAGE>   10

THE SPECIAL MEETING OF ANESTA STOCKHOLDERS (PAGE 15)

     Time, Date and Place. The special meeting of the stockholders of Anesta
will be held on October 10, 2000, at the principal executive offices of Anesta
located at 4745 Wiley Post Way, Salt Lake City, Utah 84116, at 10:00 a.m. local
time, to consider and vote on the proposal to approve the merger agreement.

     Record Date and Voting Power for Anesta. You are entitled to vote at the
special meeting if you owned shares of Anesta common stock at the close of
business on September 5, 2000, which is the record date for the special meeting.
You will have one vote at the special meeting for each share of Anesta common
stock you owned at the close of business on the record date. There are
13,451,716 shares of Anesta common stock entitled to be voted at the special
meeting.

     Anesta Required Vote. The approval of the merger agreement requires the
affirmative vote of a majority of the shares of Anesta common stock outstanding
at the close of business on the record date.

     Share Ownership of Management. At the close of business on the record date,
the directors and executive officers of Anesta own approximately 8.91% of the
shares entitled to vote at the special meeting.

COMPARATIVE PER SHARE MARKET PRICE INFORMATION (PAGE 12)

     Cephalon and Anesta common stock are both listed on the Nasdaq National
Market under the symbols "CEPH" and "NSTA," respectively. On July 14, 2000, the
last full trading day prior to the public announcement of the proposed merger,
Cephalon common stock closed at $66.00 per share and Anesta common stock closed
at $22.125 per share. On September 5, 2000, Cephalon common stock closed at
$48.50 per share and Anesta common stock closed at $22.875 per share.

     The stock prices of both Cephalon and Anesta can fluctuate broadly even
over short periods of time. It is impossible to predict the actual price of
Cephalon or Anesta common stock prior to the effective time of the merger or at
any other time.

TAX MATTERS (PAGE 31)

     We expect that, for U.S. federal tax purposes, your exchange of Anesta
common shares for Cephalon common shares in the merger generally will not cause
you to recognize any gain or loss. You will, however, recognize gain or loss in
connection with any cash received for any fractional share. Your tax basis in
the total number of shares of Cephalon common stock that you receive in the
merger will equal your current tax basis in the total number of shares of Anesta
common stock that you own (reduced by the basis allocable to any fractional
share interest for which you receive cash).

     TAX MATTERS CAN BE COMPLICATED, AND THE TAX CONSEQUENCES OF THE MERGER TO
YOU WILL DEPEND ON THE FACTS OF YOUR OWN SITUATION. YOU SHOULD CONSULT YOUR OWN
TAX ADVISOR TO FULLY UNDERSTAND THE TAX CONSEQUENCES OF THE MERGER TO YOU.

ANTICIPATED ACCOUNTING TREATMENT (PAGE 33)

     The merger is expected to be accounted for as a "pooling of interests"
transaction for financial accounting purposes. This means that Cephalon will
restate its financial statements for prior periods at the effective time of the
merger to include the assets, liabilities, stockholders' equity and results of
operation of Anesta as if Cephalon and Anesta had been combined for accounting
and financial reporting purposes since the beginning of the periods presented.

                                        5
<PAGE>   11

CONDITIONS TO THE MERGER (PAGE 39)

     The completion of the merger depends on the satisfaction or waiver of a
number of conditions, including the following:

     - approval of the merger agreement and the related transactions by the
       Anesta stockholders;

     - expiration or termination of the waiting period under the
       Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended;

     - absence of any court order, law or governmental action prohibiting the
       merger;

     - listing on the Nasdaq National Market of the Cephalon stock to be issued
       to Anesta stockholders in the merger;

     - receipt of an opinion of counsel to Anesta that the merger will qualify
       as a tax-free reorganization;

     - receipt of letters from the independent public accountants of Cephalon
       and Anesta, dated as of the closing date, that the independent public
       accountants concur with the opinion of management that the merger will
       qualify for pooling of interests accounting treatment;

     - accuracy of representations and warranties of each party except for
       inaccuracies that would not have a material adverse effect; and

     - no change or event occurs that has a material adverse effect on the other
       party.

TERMINATION OF THE MERGER AGREEMENT (PAGE 40)

     The boards of directors of Cephalon and Anesta can mutually consent to
terminate the merger agreement at any time prior to consummation of the merger.
The merger agreement can also be terminated by either company under the
following circumstances, among others:

     - the merger is not consummated by December 31, 2000;

     - approval of Anesta's stockholders is not obtained;

     - a governmental authority permanently prohibits the merger;

     - the other party breaches any of its representations or warranties under
       the merger agreement and this breach would result in a material adverse
       effect; and

     - the other party breaches in any material respect any of its obligations
       under the merger agreement.

     Cephalon may terminate the merger agreement if:

     - the Anesta board of directors does not recommend the merger to Anesta
       stockholders or withdraws or modifies its approval or recommendation of
       the merger agreement in any material manner; and

     - the Anesta board of directors recommends to Anesta stockholders any
       acquisition proposal (as defined in the merger agreement) other than by
       Cephalon.

     Anesta may terminate the merger agreement if it determines in good faith
that an acquisition proposal is a superior proposal and Anesta has complied with
its obligations under the merger agreement regarding no solicitation.

TERMINATION FEE (PAGE 40)

     The merger agreement requires Anesta to pay Cephalon a termination fee
equal to $15 million plus up to $1 million in documented out-of-pocket expenses
if the merger agreement is terminated under certain circumstances. The merger
agreement requires Cephalon to pay Anesta up to $1 million in documented
out-of-pocket expenses if the merger agreement is terminated under certain
circumstances.

                                        6
<PAGE>   12

LIMITATION ON CONSIDERING OTHER ACQUISITION PROPOSALS (PAGE 37)

     Anesta has agreed not to consider a business combination or other similar
transaction with another party while the merger is pending unless the other
party has made an unsolicited proposal to the Anesta board of directors for a
superior transaction. If an unsolicited proposal is made, Anesta has agreed to
negotiate in good faith with Cephalon for five business days to make such
changes to the terms and conditions of the merger agreement as would enable
Anesta to proceed with the transaction as contemplated.

NO APPRAISAL RIGHTS (PAGE 16)

     Under Delaware law, Anesta stockholders will not be entitled to appraisal
rights in connection with the merger.

REGULATORY APPROVALS (PAGE 33)

     U.S. antitrust laws prohibit us from completing the merger until we have
furnished certain information and materials to the Antitrust Division of the
U.S. Department of Justice and the U.S. Federal Trade Commission and a required
waiting period has expired. On August 2, 2000, Cephalon and Anesta each filed
the required notification and report forms with the Antitrust Division and the
Federal Trade Commission. On August 24, 2000, the required waiting period was
terminated. However, the Antitrust Division and Federal Trade Commission
continue to have the authority to challenge the merger on antitrust grounds
before or after we complete the merger. Consent may also be required under
applicable foreign antitrust law or regulation. We expect that the merger will
not violate any foreign antitrust laws and that all the foreign antitrust
regulatory authorities whose approval we must seek will approve the merger.

                                        7
<PAGE>   13

                      SELECTED CONSOLIDATED FINANCIAL DATA

CEPHALON SELECTED CONSOLIDATED FINANCIAL DATA

    The following selected consolidated financial data have been derived from
the consolidated financial statements of Cephalon, Inc. as of and for each of
the five years in the period ended December 31, 1999 which have been audited by
Arthur Andersen LLP, independent public accountants. The information as of and
for the six months ended June 30, 2000 and 1999, has been derived from the
unaudited consolidated financial statements of Cephalon, Inc. This data should
be read in conjunction with Cephalon's consolidated financial statements,
including the notes thereto, and "Management's Discussion and Analysis of
Financial Condition and Results of Operation" that are contained in reports
filed with the Securities and Exchange Commission and incorporated by reference
into this document.

              (AMOUNTS IN THOUSANDS, EXCEPT PER SHARE INFORMATION)

<TABLE>
<CAPTION>
                                                       SIX MONTHS ENDED
                                                           JUNE 30,                       YEAR ENDED DECEMBER 31,
                                                      -------------------   ----------------------------------------------------
                                                        2000       1999       1999       1998       1997       1996       1995
                                                      --------   --------   --------   --------   --------   --------   --------
                                                          (UNAUDITED)
<S>                                                   <C>        <C>        <C>        <C>        <C>        <C>        <C>
STATEMENT OF OPERATIONS DATA:
Revenues:
  Product sales -- Provigil.........................  $ 28,750   $  7,252   $ 25,370   $    728   $     --   $     --   $     --
  Other revenues....................................     9,256      7,727     19,549     14,927     23,140     21,366     46,999
                                                      --------   --------   --------   --------   --------   --------   --------
                                                        38,006     14,979     44,919     15,655     23,140     21,366     46,999
                                                      --------   --------   --------   --------   --------   --------   --------
Costs and Expenses:
  Cost of product sales -- Provigil.................     5,074        839      3,250         --         --         --         --
  Research and development..........................    27,377     20,152     46,420     43,649     51,587     62,096     73,994
  Selling, general and administrative...............    28,716     23,780     50,992     30,947     36,744     28,605     15,762
                                                      --------   --------   --------   --------   --------   --------   --------
                                                        61,167     44,771    100,662     74,596     88,331     90,701     89,756
                                                      --------   --------   --------   --------   --------   --------   --------
Interest (expense) income, net......................     6,761     (1,650)    (3,014)     3,534      4,772      6,205      9,754
Gain on sale of assets..............................        --         --         --         --         --      9,845         --
                                                      --------   --------   --------   --------   --------   --------   --------
Loss before extraordinary charge....................   (16,400)   (31,442)   (58,757)   (55,407)   (60,419)   (53,285)   (33,003)
Extraordinary charge for early extinguishment of
  debt..............................................        --         --    (11,187)        --         --         --         --
                                                      --------   --------   --------   --------   --------   --------   --------
Loss................................................   (16,400)   (31,442)   (69,944)   (55,407)   (60,419)   (53,285)   (33,003)
Dividends on preferred stock........................    (4,531)        --     (3,398)        --         --         --         --
                                                      --------   --------   --------   --------   --------   --------   --------
Loss applicable to common shares....................  $(20,931)  $(31,442)  $(73,342)  $(55,407)  $(60,419)  $(53,285)  $(33,003)
                                                      ========   ========   ========   ========   ========   ========   ========
Basic and diluted loss per common share:
  Loss before extraordinary charge..................  $   (.63)  $  (1.09)  $  (2.10)  $  (1.95)  $  (2.36)  $  (2.19)  $  (1.63)
  Extraordinary charge..............................        --         --       (.38)        --         --         --         --
                                                      --------   --------   --------   --------   --------   --------   --------
                                                      $   (.63)  $  (1.09)  $  (2.48)  $  (1.95)  $  (2.36)  $  (2.19)  $  (1.63)
                                                      ========   ========   ========   ========   ========   ========   ========
Weighted average number of shares outstanding.......    33,164     28,880     29,584     28,413     25,638     24,319     20,262
</TABLE>

<TABLE>
<CAPTION>
                                                                                    DECEMBER 31,
                                                  JUNE 30,    ---------------------------------------------------------
                                                    2000        1999        1998        1997        1996        1995
                                                  ---------   ---------   ---------   ---------   ---------   ---------
<S>                                               <C>         <C>         <C>         <C>         <C>         <C>
BALANCE SHEET DATA:
Cash, cash equivalents and investments..........  $150,387    $ 201,562   $  67,346   $ 119,471   $ 146,848   $ 178,067
Total assets....................................   203,687      234,053      94,673     151,208     177,891     221,330
Long-term debt..................................    13,486       14,034      15,096      27,587      16,974      21,668
Accumulated deficit.............................  (368,066)    (347,135)   (273,793)   (218,386)   (157,967)   (104,682)
Stockholders' equity............................   160,673      158,357      57,602     100,338     137,326     180,205
</TABLE>

                                        8
<PAGE>   14

ANESTA SELECTED FINANCIAL DATA

    The following selected financial data have been derived from the financial
statements of Anesta as of and for each of the five years in the period ended
December 31, 1999 which have been audited by PricewaterhouseCoopers LLP,
independent public accountants. The information as of and for the six months
ended June 30, 2000 and 1999, has been derived from the unaudited financial
statements of Anesta. This data should be read in conjunction with Anesta's
financial statements, including the notes thereto, and "Management's Discussion
and Analysis of Financial Condition and Results of Operation" that are contained
in reports filed with the Securities and Exchange Commission and incorporated by
reference into this document.

              (AMOUNTS IN THOUSANDS, EXCEPT PER SHARE INFORMATION)

<TABLE>
<CAPTION>
                                                       SIX MONTHS ENDED
                                                           JUNE 30,                       YEAR ENDED DECEMBER 31,
                                                      -------------------   ----------------------------------------------------
                                                        2000       1999       1999       1998       1997       1996       1995
                                                      --------   --------   --------   --------   --------   --------   --------
                                                          (UNAUDITED)
<S>                                                   <C>        <C>        <C>        <C>        <C>        <C>        <C>
STATEMENT OF OPERATIONS DATA:
Revenues:
  Product sales.....................................  $  3,993   $  1,128   $  2,232   $    193   $    184   $     92   $     61
  Royalty revenues..................................        26         30         62          6          5          3        102
  Contract research/license agreements..............       652        621      4,221        476         --      1,503      1,514
                                                      --------   --------   --------   --------   --------   --------   --------
        Total revenue...............................     4,671      1,779      6,515        675        189      1,598      1,677
Operating costs and expenses:
  Cost of goods sold................................     1,023        371        671         54         52         27         19
  Royalties.........................................       124         32         34          3          3          3          3
  Research and development..........................     4,940      4,713      9,840      8,627      7,889      8,304      5,228
  Depreciation and amortization.....................       605        148        310        296        269        237        158
  Marketing, general and administrative.............     9,198      4,004      9,020      8,586      6,368      3,537      2,219
                                                      --------   --------   --------   --------   --------   --------   --------
Loss from operations................................   (11,219)    (7,489)   (13,360)   (16,891)   (14,392)   (10,510)    (5,950)
Non operating income, net...........................     1,585      2,049      3,892      1,190      1,845      1,820      1,250
Provision for income taxes..........................       (31)       (15)       (20)       (16)        (2)        --         --
                                                      --------   --------   --------   --------   --------   --------   --------
Loss before cumulative effect of change in
  accounting........................................    (9,665)    (5,455)    (9,488)   (15,717)   (12,549)    (8,690)    (4,700)
Cumulative effect of change in accounting...........        --         --         --         --         --         --     (1,041)
                                                      --------   --------   --------   --------   --------   --------   --------
Net loss............................................  $ (9,665)  $ (5,455)  $ (9,488)  $(15,717)  $(12,549)  $ (8,690)  $ (5,741)
                                                      ========   ========   ========   ========   ========   ========   ========
Basic and diluted loss per common share:
  Loss before cumulative effect of change in
    accounting......................................  $  (0.72)  $  (0.41)  $  (0.72)  $  (1.59)  $  (1.32)  $  (1.02)  $  (0.65)
  Cumulative effect of change in accounting.........        --         --         --         --         --         --      (0.15)
                                                      --------   --------   --------   --------   --------   --------   --------
Net loss per common share...........................  $  (0.72)  $  (0.41)  $  (0.72)  $  (1.59)  $  (1.32)  $  (1.02)  $  (0.80)
                                                      ========   ========   ========   ========   ========   ========   ========
Weighted average shares outstanding.................    13,373     13,171     13,227      9,898      9,500      8,499      7,177
</TABLE>

<TABLE>
<CAPTION>
                                                                                    DECEMBER 31,
                                                  JUNE 30,    ---------------------------------------------------------
                                                    2000        1999        1998        1997        1996        1995
                                                  ---------   ---------   ---------   ---------   ---------   ---------
<S>                                               <C>         <C>         <C>         <C>         <C>         <C>
BALANCE SHEET DATA:
Cash, cash equivalents, certificate of deposit
  and marketable debt securities................  $ 39,670    $  72,818   $  82,335   $  29,676   $  41,359   $  21,844
Total assets....................................    70,936       78,209      85,129      32,712      43,959      24,242
Long-term obligations, including current
  portion.......................................     2,000        2,000       1,750       2,000       1,350       1,516
Accumulated deficit.............................   (67,832)     (58,167)    (48,679)    (32,962)    (20,413)    (11,719)
Stockholders' equity............................    63,676       72,426      80,019      29,198      41,115      21,677
</TABLE>

                                        9
<PAGE>   15

              SUMMARY UNAUDITED PRO FORMA COMBINED FINANCIAL DATA

     The following table sets forth certain summary pro forma combined financial
data for Cephalon and Anesta. The pro forma amounts included in the table below
assume the consummation of the merger and are based on the "pooling of
interests" method of accounting. The information has not been audited. The
following table should be read in conjunction with the historical financial
statements of Cephalon and Anesta and the pro forma financial data included
herein under the caption "Pro Forma Combined Financial Information."

     The pro forma amounts below are presented for informational purposes only
and are not necessarily indicative of the results of operations of the combined
company that would have actually occurred had the merger been consummated as of
January 1, 1997 or of the financial condition of the combined company had the
merger been consummated as of December 31, 1999 or June 30, 2000 or of the
future results of operations or financial condition of the combined company. The
pro forma information does not reflect any synergies anticipated as a result of
the merger, in particular the elimination of costs associated with Anesta's
status as a public company and other administrative savings. We cannot be sure
that such synergies will be realized.

     Cephalon and Anesta estimate that they will incur direct transaction costs
of approximately $9.0 million associated with the merger, which will be charged
to operations in the quarter in which the merger is consummated. Merger and
integration expenses are not reflected in these summary unaudited pro forma
combined consolidated financial statements.

                                       10
<PAGE>   16

              SUMMARY UNAUDITED PRO FORMA COMBINED FINANCIAL DATA
              (AMOUNTS IN THOUSANDS, EXCEPT PER SHARE INFORMATION)

<TABLE>
<CAPTION>
                                       SIX MONTHS ENDED                     YEAR ENDED
                                     --------------------   ------------------------------------------
                                     JUNE 30,    JUNE 30,   DECEMBER 31,   DECEMBER 31,   DECEMBER 31,
                                       2000        1999         1999           1998           1997
                                     ---------   --------   ------------   ------------   ------------
<S>                                  <C>         <C>        <C>            <C>            <C>
STATEMENT OF OPERATIONS DATA:
Revenues:
  Product sales....................  $ 32,743    $  8,380     $ 27,602       $    921       $    184
  License and contract.............     9,934       8,378       23,832         15,409         23,145
                                     --------    --------     --------       --------       --------
                                       42,677      16,758       51,434         16,330         23,329
                                     --------    --------     --------       --------       --------
Operating expenses:
  Cost of product sales............     6,097       1,210        3,921             54             52
  Research and development.........    32,542      24,987       56,483         52,461         59,651
  Selling, general and
     administrative................    38,418      27,842       60,133         39,647         43,209
                                     --------    --------     --------       --------       --------
                                       77,057      54,039      120,537         92,162        102,912
                                     --------    --------     --------       --------       --------
  Loss from operations.............   (34,380)    (37,281)     (69,103)       (75,832)       (79,583)
Other income.......................     8,346         399          878          4,724          6,617
                                     --------    --------     --------       --------       --------
  Loss before provision for income
     taxes.........................   (26,034)    (36,882)     (68,225)       (71,108)       (72,966)
Provision for income taxes.........       (31)        (15)         (20)           (16)            (2)
                                     --------    --------     --------       --------       --------
Loss before extraordinary charge...   (26,065)    (36,897)     (68,245)       (71,124)       (72,968)
                                     --------    --------     --------       --------       --------
Extraordinary charge for early
  extinguishment of debt...........        --          --      (11,187)            --             --
                                     --------    --------     --------       --------       --------
  Net loss.........................   (26,065)    (36,897)     (79,432)       (71,124)       (72,968)
                                     --------    --------     --------       --------       --------
Dividends on preferred stock.......    (4,531)         --       (3,398)            --             --
                                     --------    --------     --------       --------       --------
  Net loss applicable to common
     shares........................  $(30,596)   $(36,897)    $(82,830)      $(71,124)      $(72,968)
                                     ========    ========     ========       ========       ========
Basic and diluted loss per common
  share:
  Loss per common share before
     extraordinary charge..........  $  (0.77)   $  (1.05)    $  (2.00)      $  (2.15)      $  (2.42)
  Extraordinary charge.............        --          --        (0.31)            --             --
                                     --------    --------     --------       --------       --------
                                     $  (0.77)   $  (1.05)    $  (2.31)      $  (2.15)      $  (2.42)
                                     ========    ========     ========       ========       ========
Weighted average number of shares
  outstanding......................    39,536      35,156       35,887         33,129         30,165
                                     ========    ========     ========       ========       ========
</TABLE>

<TABLE>
<CAPTION>
                                                               JUNE 30,    DECEMBER 31,
                                                                 2000          1999
                                                              ----------   ------------
<S>                                                           <C>          <C>
BALANCE SHEET DATA:
Cash, cash equivalents and investments......................  $  190,057    $ 272,340
Total assets................................................     274,624      312,262
Long-term debt..............................................      15,153       15,701
Accumulated deficit.........................................    (444,898)    (414,302)
Stockholders' equity........................................     215,349      221,783
</TABLE>

                                       11
<PAGE>   17

                  SUMMARY UNAUDITED COMPARATIVE PER SHARE DATA

     The following table sets forth per share data of:

     - Cephalon on a historical basis;

     - Anesta on a historical basis;

     - Cephalon and Anesta combined on a pro forma basis; and

     - Cephalon and Anesta combined on a pro forma basis stated on an equivalent
       Anesta basis.

     This table should be read in conjunction with the historical financial
statements and notes thereto for Cephalon and Anesta incorporated by reference
into this document and the Unaudited Pro Forma Combined Financial Information
beginning on page 44. The pro forma combined per share information is not
necessarily indicative of the combined financial position or combined results of
operations that would have been reported had the companies been combined for all
periods presented, nor do they represent a forecast of the combined financial
position or results of operations for any future period. Pro forma combined and
equivalent pro forma per share data reflect the combined results of Cephalon and
Anesta presented as though they were one company for all periods shown.

     The Anesta equivalent per share pro forma information shows the effect of
the merger from the perspective of an owner of Anesta common stock. We
calculated the Anesta equivalent information by multiplying the Cephalon and
Anesta combined pro forma per share amounts by the 0.4765 merger exchange ratio.

<TABLE>
<CAPTION>
                                                    SIX MONTHS ENDED            YEAR ENDED
                                                        JUNE 30,               DECEMBER 31,
                                                    -----------------   --------------------------
                                                     2000      1999      1999     1998       1997
                                                    -------   -------   ------   ------     ------
<S>                                                 <C>       <C>       <C>      <C>        <C>
LOSS PER SHARE FROM CONTINUING OPERATIONS:
Cephalon historical basis.........................  $(0.63)   $(1.09)   $(2.48)  $(1.95)    $(2.36)
Anesta historical basis...........................   (0.72)    (0.41)    (0.72)   (1.59)     (1.32)
Cephalon and Anesta combined on a pro forma
  basis...........................................   (0.77)    (1.05)    (2.31)   (2.15)     (2.42)
Cephalon and Anesta combined on a pro forma basis
  per Anesta equivalent common share..............   (0.37)    (0.50)    (1.10)   (1.02)     (1.15)
</TABLE>

<TABLE>
<CAPTION>
                                                               AS OF        AS OF
                                                              JUNE 30,   DECEMBER 31,
                                                                2000         1999
                                                              --------   ------------
<S>                                                           <C>        <C>
BOOK VALUE PER SHARE:
Cephalon historical basis...................................   $4.62        $4.86
Anesta historical basis.....................................    4.75         5.44
Cephalon and Anesta combined on a pro forma basis...........    5.23         5.70
Cephalon and Anesta combined on a pro forma basis per Anesta
  equivalent common share...................................    2.49         2.72
</TABLE>

                                       12
<PAGE>   18

                                  RISK FACTORS

     You should consider the following matters in conjunction with the other
information included or incorporated by reference in this document in deciding
whether to vote in favor of the merger proposal. Additionally, you should
consider factors relating to Cephalon and the operation of its business
generally that are included in Cephalon's filings with the Securities and
Exchange Commission and have been incorporated by reference into this document,
including the risk factors set forth under the heading "Management's Discussion
and Analysis of Financial Condition and Results of Operations" in the annual
report on Form 10-K of Cephalon for the year ended December 31, 1999 and in the
quarterly reports on Form 10-Q filed subsequent to December 31, 1999.

  THE EXCHANGE RATIO FOR CEPHALON COMMON STOCK TO BE RECEIVED IN THE MERGER IS
FIXED AND WILL NOT BE ADJUSTED IN THE EVENT OF ANY CHANGE IN STOCK PRICE.

     Upon completion of the merger, each share of Anesta common stock will be
exchanged for 0.4765 shares of Cephalon common stock. This conversion number is
fixed and will not be adjusted as a result of any change in the price of
Cephalon common stock. Any change in the price of Cephalon common stock will
affect the value of the consideration that Anesta stockholders receive in the
merger. Because the merger will be completed only after all the conditions to
the merger are satisfied or waived, there is no way to be sure that the price of
Cephalon common stock on the date of the Anesta stockholder meeting will be the
same as its price at the time the merger is completed. The price of Cephalon
common stock can be volatile and the price at the time that the merger is
completed may be lower than its price on the date of this document or the date
of the Anesta stockholders meeting. For example, on July 14, 2000, the date of
the announcement of the merger, the closing price of Cephalon's common stock was
$66.00 per share. However, between the period July 17, 2000 to September 5,
2000, the closing price of Cephalon common stock was as high as $71.25 and as
low as $39.25. You are encouraged to obtain current market quotations for
Cephalon common stock.

  "NO SOLICITATION" RESTRICTIONS IN THE MERGER AGREEMENT AND THE TERMINATION FEE
MAY DISCOURAGE OTHER COMPANIES FROM TRYING TO ACQUIRE ANESTA.

     While the merger agreement is in effect, subject to specified exceptions,
Anesta is prohibited from entering into or soliciting, initiating or encouraging
any inquiries or proposals that may lead to a proposal or offer for a merger or
other business combination transaction with any person other than Cephalon. In
addition, in the merger agreement, Anesta agreed to pay a termination fee to
Cephalon in specified circumstances, including circumstances where a third party
acquires or seeks to acquire Anesta. These provisions could discourage other
companies from trying to acquire Anesta even though those other companies might
be willing to offer greater value to Anesta stockholders than Cephalon has
offered in the merger agreement. The payment of the termination fee could also
have a material adverse effect on Anesta's financial condition.

  SHARES OF CEPHALON COMMON STOCK ARE SUBJECT TO DIFFERENT MARKET RISKS THAN
SHARES OF ANESTA COMMON STOCK.

     Upon completion of the merger, holders of shares of common stock of Anesta
will become holders of shares of common stock of Cephalon. The business,
strategy, financial condition, results of operations and common stock of
Cephalon differ in material respects from those of Anesta. Accordingly, holders
of shares of common stock of Cephalon are subject to different market risks than
holders of shares of Anesta common stock. For a description of and other
information about the common stock of Cephalon and the differences between the
common stock of Cephalon and the common stock of Anesta, see "Summary Unaudited
Comparative Per Share Data" on page 12, "Market Price Data and Dividend
Policies" on page 53, "Comparison of the Rights of Holders of Cephalon Common
Stock and Anesta Common Stock" on page 54 and the registration statement of
Cephalon on Form 8-A, as amended, that we have incorporated by reference and
described under "Where You Can Find More Information" on page 58. For a
description of the business, strategy, financial condition and results of
operations of Cephalon, see "The Companies -- Cephalon" on
                                       13
<PAGE>   19

page 42 and the discussions in the reports on Forms 10-K, 10-Q and 8-K that we
have incorporated by reference and described under "Where You Can Find More
Information" on page 58.

  CEPHALON AND ANESTA MAY NOT REALIZE THE POTENTIAL BENEFITS OF THE MERGER.

     While we expect that the merger will allow both companies to expand their
drug development and marketing activities and give rise to other synergies as
described in "Cephalon's Reasons for the Merger" beginning on page 20 and
"Anesta's Reasons for the Merger" on page 20, these benefits may not be
realized. Cephalon and Anesta are developing, but have not yet finalized, plans
for obtaining operating synergies after the merger. The implementation of these
plans will present challenges involving the coordination of the operations,
technologies and personnel of the two companies and may unduly divert the
attention of management and cause unanticipated liabilities and costs. The
geographically dispersed operations of the two companies may increase these
challenges.

  DIRECTORS AND OFFICERS OF ANESTA HAVE CONFLICTS OF INTEREST THAT MAY HAVE
INFLUENCED THEIR OPINIONS WITH RESPECT TO THE MERGER.

     You should be aware of potential conflicts of interest, and the benefits
available to directors and officers of Anesta when considering the Anesta
board's recommendation of the merger. The directors and officers of Anesta have
interests in the merger that are in addition to, or different from, their
interests as Anesta stockholders. The Anesta board was aware of these conflicts
of interest when it approved the merger. These interests relate to:

     - Rights to accelerated stock option vesting and receipt of certain
       benefits under severance agreements; and

     - Rights to directors' and officers' insurance coverage and to
       indemnification with respect to acts and omissions in their capacities as
       directors and officers of Anesta.

See "Interests of Anesta's Officers and Directors in the Merger" on page 29.

                          FORWARD-LOOKING INFORMATION

     This document and the documents incorporated by reference contain
forward-looking statements with respect to the merger and the financial
condition, results of operations, plans, objectives, future performance and
business of Cephalon and Anesta. In some cases, you can identify forward-looking
statements by terminology, such as "may," "will," "should," "would," "expects,"
"plans," "anticipates," "believes," "estimates," "predicts," "potential" or
"continue" or the negative of these terms or other comparable terminology. Any
expectations based on these forward-looking statements are subject to risks and
uncertainties and other important factors, including those discussed in the
"Risk Factors" section of this proxy statement/prospectus and the documents
incorporated by reference. These and many other factors could affect future
financial and operating results of Cephalon, Anesta or the combined company, and
could cause actual results to differ materially from expectations based on
forward-looking statements made in this document or elsewhere by or on behalf of
Cephalon, Anesta or the combined company.

     Anesta stockholders are cautioned not to place undue reliance on these
forward looking statements, which speak only as of the date of this document or,
in the case of documents incorporated by reference, the dates of those
documents.

     All subsequent written and oral forward-looking statements attributable to
Cephalon or Anesta or any person acting on their behalf are expressly qualified
in their entirety by the cautionary statements contained or referred to in this
section. Neither Cephalon or Anesta undertakes any obligation to release
publicly any revisions to these forward-looking statements to reflect events or
circumstances after the date of this document or to reflect the occurrence of
unanticipated events.

                                       14
<PAGE>   20

                   THE SPECIAL MEETING OF ANESTA STOCKHOLDERS

DATE, TIME AND PLACE

     The special meeting of Anesta stockholders will be held on October 10,
2000, at the principal executive offices of Anesta located at 4745 Wiley Post
Way, Salt Lake City, Utah 84116, commencing at 10:00 a.m. local time. We are
sending this proxy statement/prospectus to you in connection with the
solicitation of proxies by the Anesta board of directors for use at the Anesta
special meeting and any adjournments or postponements of the special meeting.

PURPOSE OF THE SPECIAL MEETING

     The purpose of the special meeting is to consider and vote upon a proposal
to approve the merger agreement.

RECOMMENDATION OF THE ANESTA BOARD OF DIRECTORS

     The Anesta board of directors has concluded that the proposal to approve
the merger agreement is advisable and in the best interests of Anesta and its
stockholders and has unanimously approved the merger agreement. Accordingly, the
Anesta board of directors unanimously recommends that all Anesta stockholders
vote "FOR" the approval of the merger agreement.

RECORD DATE AND VOTING POWER

     Only holders of record of Anesta common stock at the close of business on
September 5, 2000, are entitled to notice of, and to vote at, the special
meeting. There were approximately 3,000 holders of record of Anesta common stock
at the close of business on the record date, with 13,451,716 shares of Anesta
common stock issued and outstanding. Each share of Anesta common stock entitles
the holder thereof to one vote on each matter submitted for stockholder
approval.

VOTING AND REVOCATION OF PROXIES

     All properly executed proxies that are not revoked will be voted at the
special meeting and at any adjournments or postponements of the special meeting
in accordance with the instructions contained in the proxy. However, if the
special meeting is adjourned to a date after November 4, 2000, a new record date
will be set by the board of directors of Anesta and a new notice of the special
meeting will be sent to persons who hold shares of Anesta's common stock as of
the new record date.

     If a holder of Anesta common stock executes and returns a proxy and does
not specify otherwise, the shares represented by the proxy will be voted "for"
approval of the merger agreement in accordance with the recommendation of the
Anesta board of directors. An Anesta stockholder who has executed and returned a
proxy may revoke it at any time before it is voted at the special meeting by:
(a) executing and returning to Anesta's Secretary a proxy bearing a later date,
or (b) filing or transmitting to Anesta's Secretary another instrument or
transmission revoking the proxy. Attendance at the special meeting will not, by
itself, constitute a revocation of the proxy. In order to change his or her
vote, an Anesta stockholder may either submit a duly executed proxy bearing a
later date or attend the meeting and vote in person after revoking any proxy
that was previously submitted. If you hold your shares in "street name" and have
instructed your broker to vote your shares, you must follow directions received
from your broker to change those instructions.

QUORUM; REQUIRED VOTE

     The presence, in person or by proxy, at the special meeting of the holders
of a majority of the shares of Anesta common stock outstanding as of the close
of business on the record date is necessary to constitute a quorum at the
meeting. For this purpose, abstentions and broker non-votes will count toward
establishment of a quorum.

                                       15
<PAGE>   21

     The affirmative vote of the holders of a majority of the shares of Anesta
common stock outstanding as of the record date is required to approve the merger
agreement. In determining whether the proposal to approve the merger agreement
has received the requisite number of affirmative votes, abstentions and broker
non-votes will have the same effect as a vote against the proposal to approve
the merger agreement.

SOLICITATION OF PROXIES

     In addition to solicitation by mail, the directors, officers, employees and
agents of Anesta may solicit proxies from Anesta stockholders by personal
interview, telephone, telegram or otherwise. Anesta will bear the costs of the
solicitation of proxies from its stockholders, except that Cephalon and Anesta
will each pay one-half of the cost of printing this proxy statement/prospectus.
Arrangements will also be made with brokerage firms and other custodians,
nominees and fiduciaries who are record holders of Anesta common stock for the
forwarding of solicitation materials to the beneficial owners of Anesta common
stock. Anesta will reimburse these brokers, custodians, nominees and fiduciaries
for the reasonable out-of-pocket expenses they incur in connection with
forwarding solicitation materials.

NO APPRAISAL RIGHTS

     Under Delaware law, a stockholder of a corporation participating in certain
major corporate transactions may, under varying circumstances, be entitled to
appraisal rights pursuant to which such stockholder may receive cash in the
amount of the fair market value of the stockholder's shares in lieu of the
consideration the stockholder would otherwise receive in the transaction.

     Delaware law provides, however, that appraisal rights are not available
with respect to a merger by a corporation (such as Anesta) whose shares are
designated as a national market system security on an interdealer quotation
system by the National Association of Securities Dealers, Inc. if stockholders
of such corporation receive only shares of the surviving corporation or shares
of any other corporation (such as Cephalon) which are designated as a national
market system security, plus cash in lieu of fractional shares. In addition,
Delaware law provides that appraisal rights are not available to stockholders of
a corporation surviving a merger (such as Cephalon) if, among other conditions,
no vote of the stockholders of the surviving corporation is required to approve
the merger because the merger agreement does not amend the existing certificate
of incorporation, each share of the surviving corporation outstanding prior to
the merger is an identical outstanding or treasury share after the merger, and
the number of shares to be issued in the merger does not exceed 20% of the
shares of the surviving corporation outstanding immediately prior to the merger.
As a result, neither the Cephalon stockholders nor the Anesta stockholders have
appraisal rights in connection with the merger.

OTHER MATTERS

     We currently are not aware of any other business to be acted upon at either
special meeting. If, however, other matters are properly brought before either
special meeting, or any adjourned or postponed special meeting, your proxies
will have discretion to vote or act on those matters according to their best
judgment, including to adjourn the special meeting.

     Adjournments or postponements of the special meeting may be made for the
purpose of, among other things, soliciting additional proxies. Any adjournment
may be made from time to time by approval of the holders of common shares
representing a majority of the votes present in person or by proxy at the
special meeting, whether or not a quorum exists, without further notice other
than by an announcement made at the special meeting unless a new record date is
set.

                                       16
<PAGE>   22

                                   THE MERGER

GENERAL DESCRIPTION OF THE MERGER

     At the effective time, C Merger Sub will be merged with and into Anesta.
Anesta will be the surviving corporation and will continue to exist under
Delaware law as a wholly-owned subsidiary of Cephalon. In the merger, Anesta
common stock outstanding at the effective time will automatically be converted
into Cephalon common stock in the ratio of 0.4765 of a share of Cephalon common
stock for each Anesta share, with cash paid for any fractional share.

     Based on the number of shares of Cephalon common stock and Anesta common
stock outstanding as of the record date, 6,409,742 shares of Cephalon common
stock will be issuable pursuant to the merger agreement, representing
approximately 15.51% of the total Cephalon common stock to be outstanding after
such issuance. This assumes that no Anesta or Cephalon stock options are
exercised between the record date and the effective time.

BACKGROUND

     The terms and conditions of the merger agreement and the merger are the
result of arm's length negotiations between representatives of Cephalon and
representatives of Anesta. Set forth below is a summary of the background of
these negotiations.

     During 1999 and into early 2000, the management of Anesta conducted an
extensive review of strategies to expand its operations through a variety of
possible transactions or agreements, including product or technology
acquisitions. In the course of this review, Anesta was approached by a number of
outside sources, including investment banking institutions and independent
individuals that suggested a variety of possible transactions. During this
period, Anesta conducted preliminary discussions with a potential merger
candidate which did not result in any proposals by either party.

     On December 17, 1999, Anesta conducted a regularly scheduled board meeting
in Salt Lake City, Utah. The board directed the senior management of Anesta to
develop and present to Abbott Laboratories comprehensive proposals for Abbott to
increase the U.S. sales and marketing resources for Actiq. Anesta's management
initiated these discussions with Abbott in January 2000. In the course of the
negotiations with Abbott, Anesta's management expressed its desire to reacquire
the U.S. sales and marketing rights for Actiq.

     On March 13, 2000, Anesta publicly announced this reacquisition to become
effective April 2, 2000. As a result of this announcement, a number of companies
initiated contact and held discussions with Anesta regarding the possibility of
co-promoting or co-marketing Actiq. In conjunction with the Abbott transaction,
Anesta evaluated a variety of strategies for marketing and selling Actiq,
including building an internal sales organization, using a third party contract
sales organization to build a sales organization, co-marketing or co-promoting
Actiq with another company with an existing sales force with expertise in
oncology and/or pain management, and combinations of these strategies.

     On April 12, 2000, the management of Anesta completed a comprehensive OTFC
(Actiq) Partnering Status Report, including an identification of possible U.S.
partners for Actiq based upon discussions between the management of Anesta and
certain companies combined with internal analyses of companies with an oncology
and/or a pain management sales focus. This report was prepared as part of a
broader presentation which was made at the April 19 board of directors meeting,
which included management's evaluation of different scenarios for Anesta to
rapidly build an Actiq field sales force and increase market penetration of
Actiq following the Anesta reacquisition from Abbott of U.S. sales and marketing
rights for Actiq.

     On April 13, 2000, Frank Baldino, Jr., Ph.D., the Chairman and CEO of
Cephalon and Thomas King, the President and CEO of Anesta, met for the first
time in New York City to discuss the possibility of co-promoting or co-marketing
Actiq. Because the two companies had not had any previous relationship or
discussions, Mr. King and Dr. Baldino only discussed publicly available
information and general background on the respective companies. No specific
discussions were conducted at this meeting and no plan was established for
further communication between the companies.
                                       17
<PAGE>   23

     On April 19, 2000, Anesta held a regularly scheduled board meeting in Salt
Lake City, Utah. Mr. King updated the board on various discussions regarding the
possibility of co-promoting or co-marketing Actiq that had taken place and level
of interest in continuing any discussions. Ten companies, including Cephalon,
were identified as having had varying degrees of contact with Anesta. Anesta's
board directed the senior management team to continue their discussions with
these (and other) companies which had expressed an interest in Actiq.

     Following the April 19, 2000 board meeting, Anesta continued to have
discussions with various companies. The discussions primarily related to a
possible U.S. co-promotion or co-marketing arrangement for Actiq. Also,
effective in mid-April, Anesta signed a one-year agreement with a contract sales
organization to build and maintain a U.S. field sales organization to sell Actiq
directly to oncologists and cancer pain management specialists.

     In early May 2000, representatives of U.S. Bancorp Piper Jaffray informed
Anesta that they had been told that Cephalon was interested in a possible
relationship with Anesta. On May 12, 2000 both companies executed a standard,
mutual confidentiality agreement, although no non-public information was
exchanged at that time.

     On May 15, 2000, Mr. King received a follow-up letter from J. Kevin Buchi,
Senior Vice President and Chief Financial Officer of Cephalon, outlining
Cephalon's interest in further exploring a possible relationship with Anesta.

     On May 20, 2000, Mr. King responded in writing to Mr. Buchi's letter and
the companies' respective management teams initiated arrangements for a meeting
at Anesta's offices in Salt Lake City, Utah in late May.

     On May 31, 2000, certain members of management of Cephalon and Anesta met
at Anesta's offices in Salt Lake City, Utah. The meeting was attended on behalf
of Anesta by William Moeller, the Chairman of Anesta's board of directors, Mr.
King, Roger Evans, Anesta's Vice President, Finance and Administration and Jeff
Williams, Anesta's Vice President, Office of Development and on behalf of
Cephalon by Robert P. Roche, Jr., Cephalon's Senior Vice President of Sales and
Marketing and Mr. Buchi. The meeting involved discussions concerning each
company's current and pipeline products. Anesta also presented a description of
the rights it had reacquired from Abbott and the manufacturing status for
producing Actiq in the U.S.

     On June 5, 2000, Cephalon and Anesta executed another non-disclosure
agreement which contemplated a possible negotiated merger between the companies,
including a standstill agreement which prohibited each company from purchasing
the securities of the other company without permission of the target company's
board.

     On June 5 and 6, 2000, Mr. King visited the Cephalon headquarters in West
Chester, Pennsylvania for the first time. Mr. King met with various members of
the Cephalon senior management team (including Dr. Baldino, Mr. Buchi, Mr.
Roche, John E. Osborn, Senior Vice President and General Counsel, Dr. Earl W.
Henry, Cephalon's Senior Vice President of Clinical Research and Regulatory
Affairs, Dr. Jeffry Vaught, Cephalon's Senior Vice President and President,
Research and Development, and Dr. Peter E. Grebow, Cephalon's Senior Vice
President of Worldwide Business Development). Representatives of U.S. Bancorp
Piper Jaffray and BancBoston Robertson Stephens were present during certain
meetings. The members of the Cephalon senior management team presented status
updates of the various programs and projects which were ongoing in their
respective functional areas. During these meetings, Mr. King, Dr. Baldino and
Mr. Buchi conducted their first discussions concerning the possibility of a
merger between the two companies and the potential strategic value of a combined
company.

     On June 7, 2000, Mr. King informed the Anesta board of Cephalon's interest
in a potential merger during a telephonic board meeting called specifically for
this purpose. The board discussed extensively the possible benefits and risks of
a merger between the two companies. The Anesta board authorized senior
management to continue discussions with Cephalon and to expand its due diligence
review of Cephalon. A series of additional meetings were tentatively scheduled
between Cephalon and Anesta for late June 2000.

                                       18
<PAGE>   24

     Beginning on June 7, 2000 and continuing through July 14, 2000, the
companies exchanged various due diligence materials as part of the ongoing due
diligence process.

     On June 20, 2000, representatives of Arthur Andersen LLP, Cephalon's
independent accountants, visited representatives of PricewaterhouseCoopers LLP,
Anesta's independent accountants, in PricewaterhouseCoopers' Salt Lake City,
Utah offices to conduct accounting and financial due diligence.

     On June 21, 2000, Anesta conducted its regularly scheduled board meeting
and annual stockholder meeting. Mr. King presented Anesta's board with a
comprehensive update of the ongoing discussions and negotiations with Cephalon,
along with a status report of ongoing discussions with other companies. A
representative of U.S. Bancorp Piper Jaffray discussed with the board the
financial aspects of two companies, including Cephalon, that were believed to
have the most significant interest in a strategic transaction with Anesta at
that time. The discussion included background and overview information on each
company and potential valuation methodologies. Representatives of Cooley Godward
LLP, counsel to Anesta, discussed with the board their fiduciary duties in
connection with their evaluation and consideration of the proposals. Anesta's
board authorized the Anesta management team to conduct exclusive negotiations
and due diligence activities regarding a potential merger with Cephalon, subject
to reaching a certain minimum exchange ratio for Anesta's outstanding common
stock. Anesta's board also authorized management to execute an engagement letter
with U.S. Bancorp Piper Jaffray.

     From June 21 through 23, 2000, members of the management teams of each
company conducted additional due diligence and meetings at the Anesta offices in
Salt Lake City, Utah. As a result of the exchange ratio being agreed upon by the
two companies, an exclusive negotiating period agreement, which had a duration
until July 21, 2000, was signed by the two companies at the close of the
meetings on June 22, 2000.

     On June 26 and 27, 2000, Mr. King, Mr. Evans, Martha Arnold, Anesta's
Senior Vice President, Carl Accetura, Anesta's Vice President of Manufacturing
Operations, and Davis Templeton, Anesta's Vice President of Sales and Field
Operations, visited the Cephalon headquarters and met with Mr. Buchi, Dr.
Baldino, Dr. Grebow, Mr. Roche, Dr. Henry, Dr. Vaught, Robert J. Urban, Senior
Director, Technical Operations and Jon R. Wallace, Cephalon's Vice President,
Quality Assurance, to continue the due diligence process.

     On June 28, 2000, Morgan, Lewis & Bockius LLP, counsel to Cephalon,
circulated the first draft of a merger agreement to Cephalon, Anesta and
Anesta's representatives. Negotiations regarding the merger agreement between
the companies and the related documents continued periodically until July 14,
2000.

     On June 30, 2000, Anesta signed an engagement letter with U. S. Bancorp
Piper Jaffray to serve as the financial advisor to Anesta for the potential
business combination.

     On July 5 and 6, 2000, Mr. King, Dennis Coleman, Ph.D., Anesta's Vice
President of Research and Development, Paul Litka, M.D., Anesta's Vice President
of Clinical Drug Development, Kim Rogers, Ph.D., Anesta's Director, Office of
Development, and a representative of Kirton & McConkie, P.C. visited the
Cephalon headquarters and met with Drs. Vaught and Henry and Messrs. Buchi,
Osborn, Carl Savini, Cephalon's Senior Vice President of Human Resources, and
Robert T. Hrubiec, JD, Ph.D., Cephalon's Director, Intellectual Property and
Chief Patent Counsel, in order to conduct additional due diligence with respect
to the companies.

     On July 10, 2000, Messrs. King and Evans of Anesta, along with
representatives of Cooley Godward and U.S. Bancorp Piper Jaffray, met with
Messrs. Buchi and Osborn, along with representatives of Morgan, Lewis and
BancBoston Robertson Stephens, in the Morgan, Lewis offices in Philadelphia,
Pennsylvania to conduct additional negotiations regarding the terms of the
merger agreement.

     On July 11, 2000, Mr. King met with Messrs. Buchi and Savini at Cephalon's
offices in order to conduct additional due diligence activities.

     On July 14, 2000, the Anesta board of directors met in Denver, Colorado to
discuss the proposed merger. At the meeting, representatives of U.S. Bancorp
Piper Jaffray presented their financial analyses with
                                       19
<PAGE>   25

respect to the proposed merger. At the conclusion of the financial review and
their presentation, U.S. Bancorp Piper Jaffray provided the Anesta board of
directors with its oral opinion (subsequently confirmed in writing) that as of
that date and based upon certain assumptions and qualifications, the exchange
ratio in the merger agreement was fair to the holders of Anesta common stock
from a financial point of view. The Anesta board of directors and
representatives of Cooley Godward discussed the terms of the merger agreement
and related agreements, as well as the results of Anesta's due diligence
efforts. After discussion, the Anesta board of directors unanimously approved
the merger agreement and the transactions contemplated thereby and authorized
the senior management of Anesta to finalize and execute the merger agreement.

     On July 14, 2000, upon completion of all negotiations and finalization of
all agreements, the two companies executed and delivered the merger agreement
and related agreements.

     On July 17, 2000, Cephalon and Anesta issued a joint press release
announcing the transaction.

     On August 31, 2000, Anesta received an unsolicited letter from a third
party that described terms under which the third party might seek to acquire all
of the common stock of Anesta in exchange for shares of common stock of the
third party. On September 4, 2000, the Anesta board of directors met to discuss
the terms of the letter, after which Anesta held preliminary discussions with
the third party. Before substantive discussions occurred, the third party sent
written notice on September 7, 2000 to Anesta that it was withdrawing its letter
from further consideration.

REASONS FOR THE MERGER

     The following discussion of the parties' reasons for the merger contains a
number of forward-looking statements that reflect the current views of Cephalon
and/or Anesta with respect to future events that may have an effect on their
future financial performance. Forward-looking statements are subject to risks
and uncertainties. Actual results and outcomes may differ materially from the
results and outcomes discussed in the forward-looking statements. Cautionary
statements that identify important factors that could cause or contribute to
differences in results and outcomes include those discussed in "Forward Looking
Information" and "Risk Factors."

  Cephalon's Reasons for the Merger.

     The Cephalon board of directors believes that the merger will accelerate
Cephalon's revenue and earnings growth in the future and shorten its path to
profitability by acquiring marketing rights to Actiq, a product for the
treatment of breakthrough pain associated with cancer. The merger will also add
Anesta's novel drug delivery platform to Cephalon's expertise in neuroscience
and research programs in oncology and will present a significant opportunity for
the development of new products in both oncology and neurology. Cephalon's
technology platform is focused, in part, on creating innovative drug therapies
to treat many forms of cancer. The merger will complement Cephalon's commercial
integration in oncology and give it access to an important medical specialty not
presently served by Cephalon's sales force. Cephalon also expects to realize
synergies through technology transfers, a strengthened sales and marketing
presence and the elimination of redundancies.

     The foregoing discussion of information and factors considered by the
Cephalon board of directors is not intended to be exhaustive but is believed to
include all material factors considered by the Cephalon board of directors. In
view of the wide variety of factors considered by the Cephalon board of
directors, the Cephalon board of directors did not find it practicable to
quantify or otherwise assign relative weight to the specific factors considered.
In addition, the Cephalon board did not reach any specific conclusion on each
factor considered, or any aspect of any particular factor, but conducted an
overall analysis of these factors. Individual members of the Cephalon board may
have given different weight to different factors. However, after taking into
account all of the factors set forth above, the Cephalon board of directors
unanimously agreed that the merger agreement and the merger were fair to, and in
the best interests of, Cephalon and its stockholders and that Cephalon should
proceed with the merger.

                                       20
<PAGE>   26

  Anesta's Reasons for the Merger.

     The Anesta board of directors unanimously approved the terms and provisions
of the merger agreement and the transactions contemplated thereby, including the
merger, at a special meeting held on July 14, 2000. In evaluating the merger,
and arriving at its approval, the Anesta board of directors considered a number
of factors, including the factors listed below.

     - The Anesta board of directors' belief that the merger would result in the
       creation of a combined company with significantly greater resources, a
       more diversified approved product and product development portfolio, and
       greater management, sales, service and marketing capabilities than those
       of Anesta alone, and would enable the combined company to compete more
       effectively with competitors having greater resources and broader product
       offerings than Anesta alone.

     - The Anesta board of directors' belief that the merger would provide
       Anesta with the potential to expand the market presence of Actiq through
       Cephalon's sales and marketing experience and resources and its
       determination that, on its own, Anesta's more limited resources would
       limit its ability to expand the sales force and the support functions
       committed to selling Actiq without being able to spread the cost across a
       broader product line. Additionally, the Cephalon development portfolio
       includes products which would be sold to the same customer group as Actiq
       is sold.

     - The Anesta board of directors' belief that the merger would provide
       increased access to capital needed for future growth and expanded product
       development.

     - The Anesta board of directors' belief that the combined company's greater
       financial resources and stability and market presence would allow it to
       enter into collaborative relationships with third parties more easily
       than Anesta could alone both for expanded relationships with the Anesta
       OTS technology platform and for possible additional compounds to be
       incorporated into the OTS technology platform.

     - The complementary nature of the product offerings of Anesta and Cephalon,
       which the Anesta board of directors believed would improve the combined
       company's competitive position by offering a more diversified product
       line than either company could offer alone, and potential future
       synergies to be created by combining Anesta's unique drug delivery
       technology and internal scientific/formulation expertise with drugs
       currently being developed by Cephalon.

     - The possibility of using Anesta's manufacturing capabilities to
       manufacture Cephalon's products.

     - The Anesta board of directors' view that the combined company's greater
       financial stability and improved long-term prospects would enable it to
       attract and retain talented employees more easily than Anesta could
       alone.

     - The Anesta board of directors' belief that cost and operating
       efficiencies could be achieved through the integration of operations of
       Anesta and Cephalon.

     - The intent that the transaction qualify as a tax-free reorganization so
       that no taxable gain or loss would be recognized by the Anesta
       stockholders on the exchange of their shares of Anesta common stock for
       Cephalon common stock.

     - Current market conditions, historical market prices and trading
       information with respect to the Cephalon common stock, including the
       increased liquidity provided through the greater trading volume of
       Cephalon common stock.

     - Other long-term alternatives for Anesta, including the possibility of
       raising additional capital through the sale of Anesta securities,
       pursuing other transaction structures or the potential of being acquired
       by another company.

     - The Anesta board of directors' belief, based on its assessment of the
       negotiations, that a higher exchange ratio or better terms could not be
       achieved through continued negotiations with Cephalon.

                                       21
<PAGE>   27

     - The Anesta board of directors' belief that the results of financial
       analyses performed by management and U.S. Bancorp Piper Jaffray were
       consistent with the consideration to be received by Anesta stockholders
       and the relative valuations of Cephalon and Anesta in the merger.

     - The opinion of U.S. Bancorp Piper Jaffray to the effect that, as of the
       date of such opinion and subject to the considerations set forth in the
       opinion, the exchange ratio pursuant to the merger agreement was fair,
       from a financial point of view, to the holders of Anesta common stock.

     This discussion of factors considered by the Anesta Board of Directors is
not necessarily complete, but it is believed to include all material factors
considered by the Anesta Board of Directors. In view of the complexity and
variety of factors considered in connection with its evaluation of the merger,
Anesta's Board of Directors did not find it practical to and did not quantify or
otherwise assign relative weights to the specific factors considered in reaching
its determination. In addition, individual members of Anesta's board of
directors may have given different weights to different factors.

     The Anesta board of directors also considered certain risks relating to the
merger, including:

     - the risk that the benefits sought in the merger would not be fully
       achieved;

     - the risk that the merger would not be consummated;

     - the risk that the value of the Cephalon common stock will not appreciate
       or that it will decline;

     - the effect of the public announcement of the merger on Anesta's sales,
       operating results and ability to enter into arrangements with third
       parties; and

     - other risks described above under "Risk Factors" on page 13.

The Anesta board of directors believed that these risks were outweighed by the
potential benefits to be gained by the merger.

OPINION OF ANESTA'S FINANCIAL ADVISOR

     Pursuant to an engagement letter dated June 30, 2000, Anesta retained U.S.
Bancorp Piper Jaffray to act as its exclusive financial advisor and, if
requested, to render to Anesta's board of directors an opinion as to the
fairness, from a financial point of view, of the proposed exchange ratio in the
merger.

     U.S. Bancorp Piper Jaffray delivered to the board of directors of Anesta on
July 14, 2000, its oral opinion (subsequently confirmed in writing as of the
same date) that, as of that date and based upon and subject to the assumptions,
factors and limitations set forth in the written opinion and described below,
the proposed exchange ratio was fair, from a financial point of view, to Anesta
stockholders. The full text of the written opinion of U.S. Bancorp Piper
Jaffray, which sets forth the assumptions made, matters considered and
limitations on review undertaken in connection with the opinion, is attached to
this proxy statement/ prospectus and is incorporated into this proxy
statement/prospectus by this reference. THIS SUMMARY IS QUALIFIED IN ITS
ENTIRETY BY REFERENCE TO THE FULL TEXT OF THE OPINION. WE URGE THE STOCKHOLDERS
OF ANESTA TO READ THE OPINION IN ITS ENTIRETY.

     While U.S. Bancorp Piper Jaffray rendered its opinion and provided certain
analyses to the board of directors of Anesta, U.S. Bancorp Piper Jaffray was not
requested to and did not make any recommendation to the board of directors as to
the specific form or amount of the consideration to be received by Anesta
stockholders in the proposed merger, which was determined through negotiations
between Cephalon and Anesta. U.S. Bancorp Piper Jaffray's written opinion, which
was directed to Anesta's board of directors, addresses only the fairness, from a
financial point of view, of the proposed exchange ratio in the merger as of the
date of such opinion, does not address the value of a share of Cephalon common
stock or Anesta common stock, and does not address Anesta's underlying business
decision to participate in the merger. U.S. Bancorp Piper Jaffray's opinion does
not constitute a recommendation to any Anesta stockholder as to how a
stockholder should vote with respect to the merger.

                                       22
<PAGE>   28

     In arriving at its opinion, U.S. Bancorp Piper Jaffray reviewed:

     - a draft of the merger agreement dated July 11, 2000;

     - publicly available financial, operating and business information related
       to Cephalon and Anesta;

     - publicly available market and securities data of Cephalon, Anesta and
       selected public companies;

     - equity research analyst reports relating to Cephalon and Anesta;

     - to the extent publicly available, financial information relating to
       selected business combinations; and

     - internal financial information of Anesta prepared for financial planning
       purposes and furnished by Anesta management.

     In addition, U.S. Bancorp Piper Jaffray held discussions with members of
the respective senior managements of Cephalon and Anesta concerning the
financial condition, current operating results and business outlook of Cephalon,
Anesta and the combined company following the merger. U.S. Bancorp Piper Jaffray
also undertook such additional reviews, analyses and inquiries as it deemed
necessary and appropriate.

     In delivering its opinion to the board of directors of Anesta, U.S. Bancorp
Piper Jaffray prepared and delivered to the board of directors written materials
containing various analyses and other information material to the opinion. The
following is a summary of the analyses contained in the materials. Some of the
summaries of financial analyses include information presented in tabular format.
In order to fully understand the financial analyses, the tables must be read
together with the text of each summary. The tables alone do not constitute a
complete description of the financial analyses. Considering the data set forth
in the tables without considering the full narrative description of the
financial analyses, including the methodologies and assumptions underlying the
analyses, could create a misleading or incomplete view of the financial
analyses.

  Implied Value of Consideration.

     Giving effect to the proposed exchange ratio of 0.4765 and the closing
price of Cephalon common stock on the Nasdaq National Market on July 12, 2000,
of $74.75, U.S. Bancorp Piper Jaffray calculated an implied value of $35.62 for
each share of Anesta common stock. Given the number of Anesta common shares
outstanding as of May 10, 2000, and corresponding common share equivalents
outstanding, U.S. Bancorp Piper Jaffray calculated the aggregate implied equity
value of the stock consideration payable in the merger for Anesta common stock
to be approximately $515.3 million.

     U.S. Bancorp Piper Jaffray also presented the implied equity values of
Anesta common stock that would result from a $50.00 to $70.00 range of Cephalon
common stock prices. U.S. Bancorp Piper Jaffray calculated that such closing
prices would imply the following equity values:

<TABLE>
<CAPTION>
                      IMPLIED AGGREGATE   IMPLIED PER SHARE
     CEPHALON         EQUITY VALUE FOR      EQUITY VALUE
CLOSING STOCK PRICE        ANESTA            FOR ANESTA
-------------------   -----------------   -----------------
<S>                   <C>                 <C>
        $50                $344.7              $23.83
        $55                $379.2              $26.21
        $60                $413.6              $28.59
        $65                $448.1              $30.97
        $70                $482.6              $33.36
</TABLE>

     U.S. Bancorp Piper Jaffray also calculated that given the proposed exchange
ratio in the merger holders of Anesta common stock and options would receive
approximately 13.5% of the common stock and common stock equivalents in the
combined entity.

                                       23
<PAGE>   29

  Anesta Market Analysis.

     U.S. Bancorp Piper Jaffray reviewed the closing prices of Anesta common
stock on the Nasdaq National Market for the 52 week period ending July 12, 2000.
U.S. Bancorp Piper Jaffray presented the closing price information contained in
the following table:

<TABLE>
<S>                                                            <C>
Closing Price as of July 12, 2000...........................   $25.38
30 day average..............................................   $20.80
60 day average..............................................   $18.14
90 day average..............................................   $17.43
52-week high................................................   $26.75
52-week low.................................................   $ 8.06
</TABLE>

  Selected Company Analysis.

     U.S. Bancorp Piper Jaffray compared financial information relating to
Anesta to corresponding data from 10 publicly traded companies in the
biotechnology industry that U.S. Bancorp Piper Jaffray believed had technologies
and/or product development stages similar to that of Anesta. These companies
were:

     - Algos Pharmaceutical Corp.;

     - Coulter Pharmaceutical Inc.;

     - Gliatech Inc.;

     - Guilford Pharmaceuticals Inc.;

     - Kos Pharmaceuticals Inc.;

     - MGI Pharma Inc.;

     - PathoGenesis Corp.;

     - Progenics Pharmaceuticals Inc.;

     - SangStat Medical Corp.; and

     - Texas Biotechnology Corp.

     This analysis produced an average and median market capitalization for the
selected companies of $402.6 and $432.7 million.

  Premiums Paid Analysis.

     U.S. Bancorp Piper Jaffray reviewed the implied premiums paid in 18
selected business combination transactions announced since January 1, 1998. U.S.
Bancorp Piper Jaffray selected these transactions by searching SEC filings,
press releases, press reports, databases and other sources and by applying the
following criteria:

     - Completed or pending transactions;

     - Change-in-control transactions; and

     - Biotechnology transactions ranging from $150 million to $1,000 million.

     The transactions reviewed were (listed as acquiror/target, and showing the
date of announcement):

     - Guilford Pharmaceuticals, Inc/Gliatech, Inc (May 30, 2000)

     - Elan Corp PLC/The Liposome Company, Inc. (March 6, 2000)

     - King Pharmaceuticals, Inc./Medco Research, Inc. (December 1, 1999)

                                       24
<PAGE>   30

     - Baxter International, Inc./North American Vaccine, Inc. (November 18,
       1999)

     - Celltech Group PLC/Medeva PLC (November 14, 1999)

     - Millennium Pharmaceuticals, Inc./LeukoSite, Inc. (October 14, 1999)

     - MedImmune, Inc./U.S. Bioscience, Inc. (September 22, 1999)

     - Teva Pharmaceutical Industries Ltd./Copley Pharmaceutical, Inc. (August
       10, 1999)

     - Biovail Corporation International/Fuisz Technologies, Ltd. (July 26,
       1999)

     - Pharmacia & Upjohn, Inc./SUGEN (June 15, 1999)

     - Celltech Group PLC/Chiroscience Group PLC (June 15, 1999)

     - Gilead Sciences, Inc./NeXstar Pharmaceuticals, Inc. (March 1, 1999)

     - Watson Pharmaceuticals, Inc./TheraTech Inc. (October 25, 1998)

     - ALZA Corporation/SEQUUS Pharmaceuticals, Inc. (October 5, 1998)

     - Nycomed Amersham PLC/Molecular Dynamics, Inc. (August 10, 1998)

     - Mylan Laboratories, Inc./Penederm Incorporated (June 24, 1998)

     - Elan Corporation PLC/Neurex Corporation (April 29, 1998)

     - Baxter International, Inc./Somatogen, Inc. (February 24, 1998)

     The table below shows a comparison of the implied premiums paid in the
selected transactions to the implied premium that would be paid to Anesta
stockholders based on the implied value payable in the merger. The premium
calculations for Anesta stock are based upon an assumed announcement date of
July 12, 2000.

<TABLE>
<CAPTION>
                                                              COMPARATIVE
                                                                PREMIUM        CEPHALON/
                                                            ----------------    ANESTA
                                                            AVERAGE   MEDIAN    MERGER
                                                            -------   ------   ---------
<S>                                                         <C>       <C>      <C>
1 day preceding announcement.............................    34.1%     33.2%     51.6%
1 week preceding announcement............................    43.2%     39.7%     50.8%
4 weeks preceding announcement...........................    58.0%     45.1%     97.9%
</TABLE>

  Discounted Cash Flow Analysis.

     U.S. Bancorp Piper Jaffray performed a discounted cash flow analysis for
Anesta. In this analysis, U.S. Bancorp Piper Jaffray calculated the present
value of the projected future cash flows of Anesta, using financial planning
data prepared by Anesta management. U.S. Bancorp Piper Jaffray estimated a range
of theoretical values for Anesta based on the net present value of its implied
annual cash flows and a terminal value for Anesta in 2004 calculated based upon
a multiple of operating income. U.S. Bancorp Piper Jaffray applied a range of
discount rates of 25% to 35% and a range of terminal value multiples of 25x to
30x forecasted 2004 operating income. This analysis yielded the following
results:

     Implied Aggregate Equity Value of Anesta:

<TABLE>
<S>                                                            <C>
Low.........................................................   $196.2 million
Mid.........................................................   $243.7 million
High........................................................   $305.2 million
</TABLE>

  Selected Transaction Analysis.

     U.S. Bancorp Piper Jaffray compared the ratio of equity value paid,
adjusted for cash and debt (referred to as the "enterprise value"), to last
twelve months ("LTM") revenue in 15 selected business combination

                                       25
<PAGE>   31

transactions. U.S. Bancorp Piper Jaffray selected the transactions by searching
SEC filings, press releases, press reports, databases and other sources and by
applying the following criteria:

     - transactions that were announced or completed between January 1, 1998 and
       July 12, 2000;

     - transactions in which the acquiring company purchased at least 50% of the
       target with cash and/or stock;

     - target companies with similar SIC codes as Anesta; and

     - value of transactions ranging from $150 million to $1,000 million.

     The transactions selected were (listed as acquiror/target, and showing the
date of announcement):

     - Elan Corp. PLC/Neurex Corp. (April 29, 1998)

     - Mylan Laboratories Inc./Penederm Inc. (June 24, 1998)

     - ALZA Corp./SEQUUS Pharmaceuticals Inc. (October 5, 1998)

     - Watson Pharmaceuticals Inc./TheraTech Inc. (October 25, 1998)

     - Tennenbaum & Co. LLC/Whittaker Corp. (December 19, 1998)

     - Gilead Sciences Inc./NeXstar Pharmaceuticals Inc. (March 1, 1999)

     - Pharmacia & Upjohn Inc./SUGEN Inc. (July 26, 1999)

     - Biovail Corp International/Fuisz Technologies Ltd (July 26, 1999)

     - Teva Pharmaceutical Industries Limited/Copley Pharmaceutical Inc. (August
       10, 1999)

     - MedImmune, Inc./US Bioscience Inc. (September 22, 1999)

     - Millennium Pharmaceuticals Inc./LeukoSite Inc. (October 14, 1999)

     - Baxter International Inc./North American Vaccine Inc. (November 18, 1999)

     - King Pharmaceuticals Inc./Medco Research Inc. (December 1, 1999)

     - Elan Corp. PLC/Liposome Co. Inc. (March 6, 2000)

     - Guilford Pharmaceuticals Inc./Gliatech Inc. (May 30, 2000)

     The average and median LTM revenue multiples for the selected transactions
calculated by U.S. Bancorp Piper Jaffray were 22.3x and 7.7x, respectively with
ranges of 179.1x as a high and 1.8x as a low. U.S. Bancorp Piper Jaffray noted
that the multiple for Anesta based on the implied value payable in the merger
and Anesta's LTM revenue was 67.5x.

  Pro Forma Contribution Analysis.

     U.S. Bancorp Piper Jaffray analyzed the expected contributions of each of
Cephalon and Anesta to revenue, operating income and pre-tax income of the
combined company for the years ending December 31, 2001, 2002 and 2003 based on
internal financial planning data of Anesta furnished by Anesta management and
research analyst estimates for Cephalon. The analysis indicated that during
these periods Anesta would contribute to the combined entity approximately the
following percentages of such amounts:

<TABLE>
<CAPTION>
                                         YEAR ENDED          YEAR ENDED          YEAR ENDED
% CONTRIBUTION BY ANESTA              DECEMBER 31, 2001   DECEMBER 31, 2002   DECEMBER 31, 2003
------------------------              -----------------   -----------------   -----------------
<S>                                   <C>                 <C>                 <C>
Revenue.............................        25.4%               22.1%               18.1%
Operating Income....................        51.0%               21.9%               14.3%
Pre-Tax Income......................        75.0%               25.4%               19.2%
</TABLE>

                                       26
<PAGE>   32

     U.S. Bancorp Piper Jaffray noted that holders of Anesta common stock and
common stock equivalents will receive approximately 13.5% of the common stock
and common stock equivalents of the combined company.

  Comparable Run Rate Analysis.

     U.S. Bancorp Piper Jaffray analyzed the current quarter of product sales
for a selected group of seven companies and annualized the most recent quarter
of sales from each. U.S. Bancorp Piper Jaffray selected these transactions by
applying the following criteria:

     - Companies with a recently approved product with annualized sales derived
       from the most recent quarter ranging from $10 to $100 million; and

     - Companies with similar SIC codes as Anesta.

     The selected companies were:

     - Celgene Corp.

     - Cephalon, Inc.

     - Gliatech, Inc.

     - IDEC Pharmaceuticals Corp.

     - Kos Pharmaceuticals, Inc.

     - MGI Pharma, Inc.

     - PathoGenesis Corp.

     U.S. Bancorp Piper Jaffray compared each company's annualized run rate
sales to its technology value to determine a multiple of annualized run rate
sales. The average and median run rate multiple for the selected companies was
35.3x and 23.7x, respectively. U.S. Bancorp Piper Jaffray noted that the implied
run rate multiple for Anesta based on the implied value payable in the merger
was 38.7x.

  Historical Exchange Ratio Analysis.

     U.S. Bancorp Piper Jaffray analyzed the exchange ratio for this transaction
against the "implied" exchange ratio based on average historical stock prices
for Cephalon and Anesta. U.S. Bancorp Piper Jaffray examined the exchange ratios
implied by the 1 year, 6 month, 3 month and 1 month average stock prices for
Cephalon and Anesta. This analysis produced the following implied historical
exchange ratios:

<TABLE>
<CAPTION>
                                                               IMPLIED EXCHANGE RATIO
                                                               ----------------------
<S>                                                            <C>
1 year average..............................................           0.413
6 month average.............................................           0.341
3 month average.............................................           0.312
1 month average.............................................           0.334
</TABLE>

     In reaching its conclusion as to the fairness of the exchange ratio and in
its presentation to the board of directors of Anesta, U.S. Bancorp Piper Jaffray
did not rely on any single analysis or factor described above, assign relative
weights to the analyses or factors considered by it, or make any conclusion as
to how the results of any given analysis, taken alone, supported its opinion.
The preparation of a fairness opinion is a complex process and not necessarily
susceptible to partial analysis or summary description. U.S. Bancorp Piper
Jaffray believes that its analyses must be considered as a whole and that
selection of portions of its analyses and of the factors considered by it,
without considering all of the factors and analyses, would create a misleading
view of the processes underlying the opinion.

                                       27
<PAGE>   33

     The analyses of U.S. Bancorp Piper Jaffray are not necessarily indicative
of actual values or future results, which may be significantly more or less
favorable than suggested by the analyses. Analyses relating to the value of
companies do not purport to be appraisals or valuations or necessarily reflect
the price at which companies may actually be sold. No company or transaction
used in any analysis for purposes of comparison is identical to Cephalon, Anesta
or the merger. Accordingly, an analysis of the results of the comparisons is not
mathematical; rather, it involves complex considerations and judgments about
differences in the companies to which Cephalon and Anesta were compared and
other factors that could affect the public trading value of the companies.

     For purposes of its opinion, U.S. Bancorp Piper Jaffray relied upon and
assumed the accuracy, completeness and fairness of the financial statements and
other information provided to it by Cephalon and Anesta or otherwise made
available to or reviewed by it and did not assume responsibility for the
independent verification of that information. U.S. Bancorp Piper Jaffray was
informed by Anesta management that the information prepared by Anesta management
for financial planning purposes was not prepared with the expectation of public
disclosure. U.S. Bancorp Piper Jaffray relied upon the assurances of the
management of Cephalon and Anesta that the information provided to it by
Cephalon and Anesta was prepared on a reasonable basis, that the financial
planning data and other business outlook information of Anesta provided to it
reflects the best currently available estimates of Anesta's management, and that
neither management was aware of any information or facts that would make the
information provided to U.S. Bancorp Piper Jaffray incomplete or misleading.

     For purposes of its opinion, U.S. Bancorp Piper Jaffray assumed that the
final form of the merger agreement would be substantially similar to the last
draft reviewed by it, without modification of material terms or conditions, and
that the merger would be consummated in accordance with the terms described in
the agreement, without waiver by Anesta of any of the conditions to its
obligations thereunder. U.S. Bancorp Piper Jaffray also assumed that the merger
will constitute a reorganization for federal income tax purposes and will be
treated as a pooling of interests for accounting purposes. U.S. Bancorp Piper
Jaffray also assumed that, in the course of obtaining the necessary regulatory
approvals and consents for the merger, no restrictions will be imposed that
would have a material adverse effect on the contemplated benefits of the merger
to Anesta and its stockholders.

     In arriving at its opinion, U.S. Bancorp Piper Jaffray did not perform any
appraisals or valuations of any specific assets or liabilities of Cephalon or
Anesta, and was not furnished with any such appraisals or valuations. U.S.
Bancorp Piper Jaffray expressed no opinion as to the liquidation value of any
entity. U.S. Bancorp Piper Jaffray expressed no opinion as to the price at which
shares of Cephalon or Anesta common stock have traded or at which the shares of
Cephalon or Anesta may trade at any future time. The opinion necessarily was
based on information available to U.S. Bancorp Piper Jaffray and the facts and
circumstances as they existed and were subject to evaluation on the date of the
opinion. Events occurring after that date could materially affect the
assumptions used in preparing the opinion. U.S. Bancorp Piper Jaffray has not
undertaken to and is not obligated to affirm or revise its opinion or otherwise
comment on any events occurring after the date it was given. U.S. Bancorp Piper
Jaffray's analyses are based upon numerous factors and events beyond the control
of the parties or their respective advisors.

     U.S. Bancorp Piper Jaffray, as a customary part of its investment banking
business, is engaged in the valuation of businesses and their securities in
connection with mergers and acquisitions, underwritings and secondary
distributions of securities, private placements and valuations for estate,
corporate and other purposes. The board of directors of Anesta selected U.S.
Bancorp Piper Jaffray because of its expertise, reputation and familiarity with
the biotechnology industry in general and with Anesta in particular. In the
ordinary course of its business, U.S. Bancorp Piper Jaffray and its affiliates
may actively trade securities of Cephalon or Anesta for their own accounts or
the accounts of their customers and, accordingly, may at any time hold a long or
short position in those securities.

     Under the terms of the engagement letter dated June 30, 2000, Anesta has
agreed to pay U.S. Bancorp Piper Jaffray a fee equal to 1.0% of the aggregate
consideration paid to Anesta stockholders or optionholders in connection with
the merger for its financial advisory services, payable upon consummation of the
merger.

                                       28
<PAGE>   34

Anesta also has agreed to pay U.S. Bancorp Piper Jaffray $550,000 for rendering
its opinion and a retainer fee, which fees are not contingent upon consummation
of the merger and will be credited against payment of the fee for financial
advisory services. The contingent nature of the financial advisory fee may have
created a potential conflict of interest in that Anesta would be unlikely to
consummate the merger unless it had received the opinion of U.S. Bancorp Piper
Jaffray. Whether or not the transaction is consummated, Anesta also has agreed
to pay the reasonable out-of-pocket expenses of U.S. Bancorp Piper Jaffray and
to indemnify U.S. Bancorp Piper Jaffray against certain liabilities. These
liabilities include liabilities under the federal securities laws in connection
with the engagement of U.S. Bancorp Piper Jaffray by Anesta's board of
directors.

INTERESTS OF ANESTA'S OFFICERS AND DIRECTORS IN THE MERGER

     In considering the recommendation of the Anesta board of directors with
respect to approving the merger agreement, Anesta stockholders should be aware
that certain members of the board of directors and management of Anesta have
interests in the merger that are in addition to the interests of stockholders of
Anesta generally. The Anesta board of directors was aware of these interests and
considered them, among other matters, in approving the merger agreement.

  Indemnification and Insurance.

     The merger agreement provides for the survival after the merger of all
indemnification rights of the directors and officers of Anesta for acts and
omissions occurring before the merger, as their rights existed as of July 14,
2000, in the Anesta bylaws and in indemnification agreements with Anesta.
Cephalon has agreed to guarantee that the surviving company observes its
commitments under these indemnification agreements. In addition, for a period of
six years after the closing of the merger, the surviving corporation will
maintain in effect the existing policy of directors' and officers' liability
insurance policy maintained by Anesta or substitute for the current policy, a
policy or policies with comparable coverage. If the annual premiums of such
insurance coverage exceed 150% of the last annual premium paid for the insurance
prior to July 14, 2000, the surviving corporation may reduce the amount of
coverage to the amount of coverage that can be obtained for an annual premium
equal to 150% of the last annual premium paid for the insurance prior to July
14, 2000.

  Option Acceleration.

     Options granted to Anesta's officers and directors pursuant to the Anesta
stock option plans accelerate upon occurrence of the merger. Options granted to
officers that are unvested as of the date of the merger generally will
accelerate according to the following schedule: (1) 50% of the unvested portion
of an officer's options will accelerate and immediately become vested and
exercisable upon the occurrence of the merger; (2) the remaining 50% of the
officer's unvested options will vest in 12 equal monthly installments following
the date of the merger; and (3) if within 24 months following the merger the
officer's employment terminates due to an involuntary termination without cause
or a constructive termination, any unvested outstanding options will accelerate
and immediately become vested and exercisable. Options granted to directors that
are unvested as of the date of the merger generally will accelerate in full and
be immediately vested and exercisable upon occurrence of the merger.

                                       29
<PAGE>   35

     The following table summarizes the outstanding Anesta stock options held by
each person who has served as an officer or director of Anesta as of July 1,
2000:

<TABLE>
<CAPTION>
                                                                                            WEIGHTED
                                                                                             AVERAGE
                                                                                            EXERCISE
                                                                                            PRICE OF
                                                  TOTAL NUMBER    NUMBER OF                IMMEDIATELY
                                                 OF OUTSTANDING   UNVESTED    IMMEDIATE      VESTED
NAME                                                OPTIONS        OPTIONS    VESTING(1)     OPTIONS
----                                             --------------   ---------   ----------   -----------
<S>                                              <C>              <C>         <C>          <C>
Thomas King....................................     332,500        126,046      63,023       $12.80
Martha Arnold..................................      84,584         37,133      18,567        12.95
Frank Kiser....................................      76,000         38,044      19,022        13.00
Paul Litka.....................................      73,500         40,699      20,350        14.60
Dennis Coleman.................................      71,000         31,795      15,898        12.74
Jeffrey Williams...............................      63,616         41,139      20,570        12.23
Carl Accetura..................................      73,500         43,824      21,912        12.38
W. Davis Templeton.............................      64,501         29,200      14,600        12.67
Roger Evans....................................      61,418         35,982      17,991        12.53
Steven Shoemaker...............................      71,000         31,795      15,898        12.73
William Moeller................................      65,731          5,211       5,211        13.00
Richard Urfer..................................      35,600         16,894      16,894        16.76
Richard Leazer.................................      33,500         18,794      18,794        17.41
Daniel Kisner..................................      41,600         18,394      18,394        17.45
Emmanuel Papper................................      24,300         12,130      12,130        17.61
Theodore Stanley...............................      38,500         12,034      12,034        13.26
</TABLE>

---------------

(1) Number immediately vested as if the merger were to close on July 1, 2000.

  Severance Benefits.

     In addition to the acceleration of the vesting of stock options, certain
officers may receive certain other severance benefits upon occurrence of the
merger. If within 24 months following the merger such officer's employment
terminates due to an involuntary termination without cause or a constructive
termination, the officer generally will receive a severance payment equal to 12
months of pay (24 months in the case of Mr. King), payable in a lump sum or
installments over not more than 12 months. Such officer also will be eligible
upon termination to receive health care continuation benefits pursuant to the
Consolidated Omnibus Reconciliation Act of 1985, or COBRA, and Cephalon
generally will continue to pay its portion of the officer's COBRA insurance
premium for up to 12 months after the date of termination. Additionally,
terminated officers generally will be entitled to receive any accrued vacation
pay in accordance with Anesta's vacation pay policy.

     The following table sets forth the severance payments each officer will be
eligible for if they are terminated:

<TABLE>
<CAPTION>
NAME                                                            AMOUNT
----                                                           --------
<S>                                                            <C>
Thomas King.................................................   $634,130
Paul Litka..................................................    245,700
Martha Arnold...............................................    240,068
Steven Shoemaker............................................    239,101
Franck Kiser................................................    219,625
Jeffrey Williams............................................    216,466
Dennis Coleman..............................................    210,600
Carl Accetura...............................................    209,213
W. Davis Templeton..........................................    183,934
Roger Evans.................................................    179,325
</TABLE>

                                       30
<PAGE>   36

MATERIAL FEDERAL INCOME TAX CONSEQUENCES

     The following discussion summarizes the anticipated material U.S. federal
income tax consequences of the merger to Anesta stockholders who exchange their
Anesta common stock for Cephalon common stock in the merger. This summary is
based upon current provisions of the Internal Revenue Code, existing regulations
under the Internal Revenue Code and current administrative rulings and court
decisions, all of which are subject to change. Any such change, which may or may
not be retroactive, could alter the tax consequences to Cephalon, Anesta or the
stockholders of Anesta described in this summary. No attempt has been made to
comment on all federal income tax consequences of the merger that may be
relevant to particular Anesta stockholders, including stockholders:

     - who are subject to special tax rules, such as dealers in securities,
       foreign persons, mutual funds, insurance companies and tax-exempt
       entities;

     - who are subject to the alternative minimum tax provisions of the Internal
       Revenue Code;

     - who acquired their shares in connection with stock option or stock
       purchase plans or in other compensatory transactions;

     - who hold their shares as a hedge or as part of a hedging, straddle or
       other risk reduction strategy; or

     - who do not hold their shares as capital assets.

     In addition, the following discussion does not address the tax consequences
of the merger under state, local and foreign tax laws. Furthermore, the
following discussion does not address:

     - the tax consequences of transactions effectuated before, after or at the
       same time as the merger, whether or not they are in connection with the
       merger, including, without limitation, transactions in which Anesta
       shares are acquired or Cephalon shares are disposed of;

     - the tax consequences to holders of options issued by Anesta that are
       assumed, exercised or converted, as the case may be, in connection with
       the merger; or

     - the tax consequences of the receipt of Cephalon shares other than in
       exchange for Anesta shares.

     Accordingly, holders of Anesta common stock are advised and expected to
consult their own tax advisors regarding the federal income tax consequences of
the merger in light of their personal circumstances and the consequences under
state, local and foreign tax laws.

     Morgan, Lewis & Bockius LLP has delivered to Cephalon, and Cooley Godward
LLP has delivered to Anesta, an opinion stating that the merger will constitute
a reorganization within the meaning of Section 368(a) of the Internal Revenue
Code. These opinions are attached as Exhibits 8.1 and 8.2 to the registration
statement on Form S-4 filed with the Securities and Exchange Commission, which
includes this proxy statement/prospectus. The tax opinions discussed in this
section assume and are conditioned upon the following:

     - the truth and accuracy of the statements, covenants, representations and
       warranties contained in the merger agreement, in the tax representations
       received from Cephalon, C Merger Sub and Anesta to support the tax
       opinions and in all other instruments and documents related to the
       formation, organization and operation of Cephalon, C Merger Sub and
       Anesta examined by and relied upon by Cooley Godward LLP and Morgan,
       Lewis & Bockius LLP in connection with the merger;

     - that original documents submitted to such counsel are authentic, that
       documents submitted to such counsel as copies conform to the original
       documents and that all of these documents have been (or will be by the
       effective time) duly and validly executed and delivered where due
       execution and delivery are a prerequisite to the effectiveness of these
       documents;

                                       31
<PAGE>   37

     - that all covenants contained in the merger agreement and the tax
       representations described above are performed without waiver or breach of
       any material provision of these covenants; and

     - that any representation or statement made "to the best of knowledge" or
       similarly qualified is correct without that qualification.

     No ruling from the Internal Revenue Service has been or will be requested
in connection with the merger. In addition, stockholders of Anesta should be
aware that the tax opinions discussed in this section are not binding on the IRS
or any court. The IRS could adopt a contrary position and a contrary position
could be sustained by a court.

     Subject to the assumptions and limitations discussed above, it is the
opinion of Morgan, Lewis & Bockius LLP, tax counsel to Cephalon, and Cooley
Godward LLP, tax counsel to Anesta, that:

     - the merger will be treated for federal income tax purposes as a
       reorganization;

     - Cephalon, C Merger Sub and Anesta will each be a party to the
       reorganization;

     - Cephalon, C Merger Sub and Anesta will not recognize any gain or loss
       solely as a result of the merger;

     - stockholders of Anesta will not recognize any gain or loss upon the
       receipt of solely Cephalon common stock (including any rights which may
       attach to such common stock pursuant to Cephalon's Amended and Restated
       Rights Agreement, dated as of January 1, 1999 and as amended on July 31,
       2000) for their Anesta common stock;

     - the aggregate basis of the shares of Cephalon common stock and rights
       under Cephalon's Amended and Restated Rights Agreement that are received
       by an Anesta stockholder in the merger (including any fractional share
       deemed received) will be the same as the aggregate basis of the shares of
       Anesta common stock surrendered in exchange therefor;

     - the holding period of the shares of Cephalon common stock received by an
       Anesta stockholder in the merger will include the holding period of the
       shares of Anesta common stock surrendered in the merger if such shares of
       Anesta common stock are held as capital assets at the effective time of
       the merger; and

     - a stockholder of Anesta who receives cash instead of a fractional share
       of Cephalon common stock will recognize gain or loss equal to the
       difference, if any, between such stockholder's basis in the fractional
       share and the amount of cash received. Such gain or loss will be a
       capital gain or loss if the Anesta common stock is held by such
       stockholder as a capital asset at the effective time of the merger. The
       deductibility of capital losses is subject to limitations for both
       individuals and corporations.

     In addition to the foregoing, there are other tax-related issues that you
should be aware of, such as:

     - Reporting Requirements. Each Anesta stockholder that receives Cephalon
       common stock in the merger will be required to file a statement with his,
       her or its federal income tax return setting forth the stockholder's
       basis in the Anesta stock surrendered and the fair market value of the
       Cephalon stock and cash received in the merger, and to retain permanent
       records of these facts relating to the merger.

     - Backup Withholding. Unless an exemption applies under applicable law and
       regulations, the exchange agent is required to withhold, and will
       withhold, 31% of any cash payments to an Anesta stockholder in the merger
       unless the stockholder provides the appropriate form as described below.
       Each Anesta stockholder should complete and sign the substitute Form W-9
       included as part of the letter of transmittal to be sent to each Anesta
       stockholder, so as to provide the information, including such
       stockholder's taxpayer identification number, and certification necessary
       to avoid backup withholding,

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<PAGE>   38

       unless an applicable exemption exists and is proved in a manner
       satisfactory to Cephalon and the exchange agent.

     - Consequences of IRS Challenge. A successful IRS challenge to the
       reorganization status of the merger would result in significant tax
       consequences. Anesta stockholders would recognize gain or loss with
       respect to each share of Anesta common stock surrendered in the merger.
       Such gain or loss would be equal to the difference between the
       stockholder's basis in such share and the sum of the fair market value,
       as of the effective time, of the Cephalon common stock received in the
       merger and any cash received instead of a fractional share of Cephalon
       common stock. In such event, a stockholder's aggregate basis in the
       Cephalon common stock so received would equal its fair market value as of
       the effective time and the stockholder's holding period for such stock
       would begin the day after the merger is consummated.

     - Other Consideration. Even if the merger qualifies as a reorganization, a
       recipient of Cephalon common stock would recognize income if, for
       example, any such shares were determined to have been received in
       exchange for services, to satisfy obligations or in consideration for
       anything other than the Anesta capital stock surrendered. Generally, such
       income is taxable as ordinary income upon receipt. In addition, to the
       extent that Anesta stockholders were treated as receiving, directly or
       indirectly, consideration other than Cephalon capital stock in exchange
       for such stockholder's Anesta common stock, gain or loss would have to be
       recognized.

     THE SUMMARY OF MATERIAL U.S. FEDERAL INCOME TAX CONSEQUENCES SET FORTH
ABOVE IS INTENDED TO PROVIDE ONLY A GENERAL SUMMARY AND IS NOT INTENDED TO BE A
COMPLETE ANALYSIS OR DESCRIPTION OF ALL POTENTIAL FEDERAL INCOME TAX
CONSEQUENCES OF THE MERGER. IN ADDITION, THE SUMMARY DOES NOT ADDRESS TAX
CONSEQUENCES THAT MAY VARY WITH, OR ARE CONTINGENT ON, INDIVIDUAL CIRCUMSTANCES.
MOREOVER, THE SUMMARY DOES NOT ADDRESS ANY NON-INCOME TAX OR ANY FOREIGN, STATE
OR LOCAL TAX CONSEQUENCES OF THE MERGER. THE SUMMARY DOES NOT ADDRESS THE TAX
CONSEQUENCES OF ANY TRANSACTION OTHER THAN THE MERGER. ACCORDINGLY, EACH ANESTA
STOCKHOLDER IS STRONGLY URGED TO CONSULT WITH A TAX ADVISOR TO DETERMINE THE
PARTICULAR FEDERAL, STATE, LOCAL OR FOREIGN INCOME OR OTHER TAX CONSEQUENCES OF
THE MERGER TO SUCH STOCKHOLDER.

ANTICIPATED ACCOUNTING TREATMENT

     The merger is expected to be accounted for using the "pooling of interests"
method of accounting pursuant to Opinion No. 16 of the Accounting Principles
board. The pooling of interests method of accounting assumes that the combining
companies have been merged from inception, and the historical consolidated
financial statements for periods prior to completion of the merger are restated
as though the companies had been combined from inception. Cephalon may terminate
the merger agreement if the merger cannot be accounted for as a pooling of
interests.

REGULATORY APPROVALS

     Under the Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended,
and the rules promulgated thereunder (the "HSR Act"), the merger may not be
completed until notifications have been given and certain information has been
furnished to the Federal Trade Commission (the "FTC") and the Antitrust Division
of the U.S. Department of Justice and specified waiting period requirements have
been satisfied or early termination of the waiting period is granted at the
request of Cephalon and Anesta. Any consent required under any applicable
foreign antitrust law or regulation also must have been obtained. Cephalon and
Anesta completed their filing of the notification and report forms under the HSR
Act with the FTC and the Antitrust Division on August 2, 2000. These filings
commenced a 30-day waiting period under the HSR Act that was terminated early by
the FTC on August 24, 2000.

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<PAGE>   39

     There can be no assurance that a challenge to the merger on antitrust
grounds will not be made, or if such challenge is made, that Cephalon and Anesta
would prevail or would not be required to terminate the merger agreement, to
divest certain assets, to license certain proprietary technology to third
parties or to accept certain conditions in order to complete the merger.

RESTRICTIONS ON RESALE BY AFFILIATES

     The shares of Cephalon common stock to be received by Anesta stockholders
in the merger have been registered under the Securities Act and, except as
described in this paragraph, may be freely traded without restriction. The
shares of Cephalon common stock to be issued in the merger and received by
persons who are considered to be "affiliates" (as that term is used in Rule 145
under the Securities Act) of Anesta before the merger may be resold by them only
in transactions permitted by the resale provisions of Rule 145 under the
Securities Act or as otherwise permitted under the Securities Act. Under
guidelines published by the SEC, the sale or other disposition of Cephalon
common stock or Anesta common stock by an affiliate of either Cephalon or Anesta
within 30 days before the effective time of the merger or the sale or other
disposition of Cephalon common stock after the merger and before the publication
of financial results that include at least 30 days of post-merger combined
operations of Cephalon and Anesta could preclude pooling of interests accounting
treatment of the merger. Accordingly, Cephalon has obtained signed affiliate
agreements from all persons whom it has been advised may be considered to be
affiliates of Anesta. The affiliate agreements provide that these persons will
not sell, transfer or otherwise dispose of any shares of Anesta common stock or
Cephalon common stock during the period referred to above and that they will not
sell, transfer or otherwise dispose of Cephalon common stock at any time in
violation of the Securities Act, including Rule 145.

                     CERTAIN TERMS OF THE MERGER AGREEMENT

     The following is a summary of the material terms of the merger agreement
among Cephalon, C Merger Sub and Anesta, and is not an exhaustive description.
You should read the merger agreement carefully and in its entirety. A copy of
the merger agreement is attached as Annex A to this proxy statement/prospectus
and is incorporated herein by reference.

GENERAL

     Under the merger agreement, C Merger Sub, Inc., a wholly-owned subsidiary
of Cephalon, will merge with and into Anesta, with Anesta continuing as the
surviving corporation and wholly owned subsidiary of Cephalon. Following the
merger, Anesta will be led by Cephalon's management team.

CLOSING MATTERS

  Closing.

     Unless the parties agree otherwise, the closing of the merger will take
place as soon as practicable, but no later than two business days after all
closing conditions have been satisfied or waived, unless the merger agreement
has been terminated. See "Conditions to Obligations to Effect the Merger" below
for a more complete description of the conditions that must be satisfied prior
to closing.

  Effective Time.

     On the closing date, Cephalon and Anesta will file a certificate of merger
with the Delaware Secretary of State in accordance with the relevant provisions
of the Delaware General Corporation Law. The merger will become effective when
the certificate of merger is filed or at such later time as Cephalon and Anesta
agree and specify in the certificate of merger.

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<PAGE>   40

CONVERSION OF SHARES; TREATMENT OF STOCK OPTIONS

     As a result of the merger, each share of Anesta's common stock issued and
outstanding immediately prior to the effective time will be converted into the
right to receive 0.4765 shares of Cephalon common stock, par value $.01 per
share. If you own Anesta common stock, you may exchange your shares of Anesta
common stock for shares of Cephalon common stock after the merger becomes
effective. In addition, the exchange agent will make a cash payment to you for
any fractional share of Cephalon common stock you would otherwise be entitled to
receive.

     At the effective time of the merger, each outstanding and unexercised
option or right to purchase shares of Anesta common stock granted under the
Anesta stock option plans will be converted into an option or a right to
purchase shares of Cephalon common stock under the same terms and conditions as
were applicable to the options as granted under the Anesta stock plans. The
number of shares of Cephalon common stock that the converted options will be
exercisable for, and the exercise price of the option, will be adjusted to
reflect the exchange ratio of 0.4765.

     Each share of Cephalon common stock will remain outstanding following the
merger and will continue to represent one share of common stock of the combined
company.

EXCHANGE OF STOCK CERTIFICATES

     Before the closing of the merger, Cephalon will appoint an exchange agent
to handle the exchange of Anesta stock certificates for stock of Cephalon and
the payment of cash for fractional shares. Soon after the closing of the merger,
the exchange agent will send a letter of transmittal to each former Anesta
stockholder, which is to be used to exchange Anesta stock certificates for stock
of Cephalon. The letter of transmittal will contain instructions explaining the
procedure for surrendering Anesta stock certificates.

     Anesta stockholders who surrender their stock certificates, together with a
properly completed letter of transmittal, will receive shares of Cephalon common
stock into which the shares of Anesta common stock were converted in the merger.

     After the merger, each certificate that previously represented shares of
Anesta stock will only represent the right to receive the shares of Cephalon
common stock into which those shares of Anesta common stock have been converted.

     After the effective time of the merger, Anesta will not register any
transfers of the shares of Anesta common stock.

REPRESENTATIONS AND WARRANTIES

     The merger agreement contains representations and warranties of Anesta that
are customary for a transaction of this nature relating to, among other things:

     - Anesta's organization, capitalization and authority to enter into the
       merger agreement;

     - the enforceability of the merger agreement as a binding obligation of
       Anesta; any conflicts between the merger agreement and any of Anesta's
       other material contracts, any law, or any of Anesta's charter or bylaw
       provisions; any required filings and consents;

     - the accuracy of Anesta's financial statements and filings with the SEC
       and Anesta's submission of all required filings;

     - material liabilities or obligations incurred by Anesta or any Anesta
       subsidiary since December 31, 1999, other than in the ordinary course of
       business;

     - indebtedness and absence of undisclosed liabilities;

     - the absence of litigation;

     - compliance with laws;

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<PAGE>   41

     - environmental matters;

     - Anesta's conduct of its business since December 31, 1999 and the absence
       of any material adverse effect on the business of Anesta;

     - tax matters;

     - employee benefit and labor matters;

     - intellectual property matters;

     - certain matters with respect to pooling of interests accounting;

     - brokers and finders in connection with the merger;

     - Anesta's financial advisor's opinion;

     - insurance;

     - the validity of Anesta's material contracts;

     - title to Anesta's owned and leased personal property;

     - Anesta's owned and leased real property;

     - supply and distribution relationships;

     - regulatory matters; and

     - facts or circumstances that would initially affect the continued
       marketing, manufacture, and sale of Actiq as currently conducted.

     The merger agreement also includes representations and warranties by
Cephalon as to:

     - Cephalon's organization, capitalization and authority to enter into the
       merger agreement;

     - the enforceability of the merger agreement as a binding obligation of
       Cephalon;

     - any conflicts between the merger agreement and any of Cephalon's other
       material contracts, any law, or any of Cephalon's charter or bylaw
       provisions;

     - required filings and consents;

     - the accuracy of Cephalon's financial statements and filings with the SEC
       and Cephalon's submission of all required filings;

     - material liabilities or obligations incurred by Cephalon or any Cephalon
       subsidiary since December 31, 1999 other than in the ordinary course of
       business;

     - the absence of litigation;

     - compliance with laws;

     - Cephalon's conduct of its business since December 31, 1999 and the
       absence of any material adverse effect on Cephalon;

     - certain matters with respect to pooling of interests accounting;

     - brokers and finders in connection with the merger; and

     - Cephalon's financial advisor's opinion.

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<PAGE>   42

COVENANTS

     The merger agreement contains the following covenants made by Cephalon and
Anesta, among others.

  No Solicitation:

     Anesta agreed not to, without the prior written consent of Cephalon:

     - solicit, initiate or encourage (including by way of furnishing certain
       information) or take any other action to facilitate knowingly any
       inquiries or the making of any proposal that would constitute or may
       reasonably be expected to lead to an acquisition proposal;

     - engage in any discussion with any third party other than its
       representatives, or negotiations relating to any acquisition proposal; or

     - enter into any agreement with respect to, agree to, approve or recommend
       any acquisition proposal.

     An acquisition proposal is a proposal or offer for a tender or exchange
offer, merger, consolidation or other business combination involving Anesta or
any Anesta subsidiary or any proposal to acquire in any manner a substantial
equity interest in, or a substantial portion of the assets of, Anesta or any
Anesta subsidiary.

     This covenant does, however, expressly permit Anesta to:

     - prior to the approval of the merger agreement by Anesta's stockholders,
       engage in discussions or negotiations with a third party who (without
       solicitation or initiation by Anesta) makes an unsolicited bona fide
       written acquisition proposal, if the Anesta board of directors
       determines, in good faith after consultation with its financial advisors
       and independent legal counsel, that such competing proposal is
       financially superior to the transactions contemplated by the merger
       agreement and that in the case of a cash offer, such third party is able
       to finance the acquisition proposal and after considering applicable
       provisions of state law, that such action is necessary for the board of
       directors to act in a manner consistent with its fiduciary duties under
       applicable law;

     - comply with 14e-2 of the Exchange Act with regard to a tender or exchange
       offer; and

     - accept a superior proposal.

     In the event that Anesta accepts a superior proposal, it will be obligated
to pay a termination fee of $15,000,000 plus expenses up to $1,000,000. See
"Termination; Termination Fees and Expenses" on page 40.

     In addition, Anesta agreed to notify Cephalon promptly if it receives any
competing proposal or inquiries indicating that any person is considering making
a competing proposal. Additionally, prior to accepting a superior proposal,
Anesta agreed to negotiate in good faith with Cephalon, for a period of not less
than five business days, to make such changes to the terms and conditions of the
merger agreement that would enable Anesta to proceed with the transactions
contemplated by the merger agreement.

  Conduct of Anesta's Business.

     Anesta made covenants concerning the conduct of its business and the
business of its subsidiaries from the date of execution of the merger agreement
until the effective time of the merger. In general, Anesta is required to
conduct its business in the ordinary course in the same manner as previously
conducted and to use its commercially reasonable efforts to preserve intact its
present relationships with customers, suppliers and other third parties. Anesta
has agreed that it will not do any of the following without Cephalon's prior
written consent:

     - amend or propose to amend its charter or bylaws;

     - change its capitalization including (a) stock splits, combinations or
       reclassifications, (b) repurchasing or redeeming its capital stock and
       (c) issuing, delivering or selling any of its shares of its capital stock
       or other equity interests, other than in connection with its benefit
       plans or the exercise of options;

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<PAGE>   43

     - acquire, merge or consolidate with or into any other corporation or other
       business organization or make any investment therein;

     - incur indebtedness other than in the ordinary course of business, with
       certain specified exceptions;

     - release or relinquish any material contract rights;

     - make certain tax elections, settlements or compromises;

     - make salary, wage and benefits increases other than in the ordinary
       course of business; and

     - take any action that would cause the merger not to be treated as a
       "pooling of interests" for accounting purposes and a reorganization
       within the meaning of Section 368(a) of the Code.

  Conduct of Cephalon's Business.

     Cephalon made covenants concerning the conduct of its business and the
business of its subsidiaries from the date of execution of the merger agreement
until the effective time of the merger. In general, Cephalon is required to
conduct its business in the ordinary course in the same manner as previously
conducted. Cephalon has agreed that it will not do any of the following without
Anesta's prior written consent:

     - amend or propose to amend its charter or bylaws;

     - change its capitalization including (a) stock splits, combinations or
       reclassifications, (b) repurchasing or redeeming its capital stock and
       (c) issuing, delivering or selling any of its shares of its capital stock
       or other equity interests, other than in connection with its benefit
       plans or the exercise of options;

     - take any action that would cause the merger not to be treated as a
       "pooling of interests" for accounting purposes and a reorganization
       within the meaning of Section 368(a) of the Code.

  Stockholders' Meeting.

     Anesta agreed to use its reasonable best efforts to solicit and secure from
its stockholders the approval and adoption of the merger agreement and the
merger subject to the fiduciary duties of its board of directors under
applicable law.

  Reasonable Efforts.

     Each party agreed to use its commercially reasonable efforts to take all
actions and to do all things necessary, proper or advisable under applicable
laws, statutes, ordinances, codes, rules and regulations to consummate and make
effective the transactions contemplated by the merger agreement in the most
expeditious manner practicable.

  Access to Information.

     Cephalon and Anesta each agreed to provide the other reasonable access to
its and its subsidiaries' facilities, books and records and to furnish certain
information to the other upon reasonable request. Cephalon and Anesta each
agreed to comply with all of their respective obligations under a
confidentiality agreement, dated as of June 5, 2000, between Cephalon and
Anesta.

  Public Announcements.

     The parties agreed to consult with each other before making any public
announcements or otherwise communicating with any news media concerning the
merger agreement or the transactions contemplated thereby, unless applicable law
requires.

  Directors' and Officers' Indemnification and Insurance.

     Cephalon agreed to maintain in effect the current provisions regarding
indemnification of officers and directors contained in the charter documents and
bylaws of Anesta. Cephalon further agreed that it shall cause the surviving
corporation to maintain, for six years following the effective time of the
merger, policies of directors' and officers' liability insurance that are no
less advantageous than those provided to directors and

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<PAGE>   44

officers of Anesta immediately prior to the effective time of the merger;
provided, that Cephalon is not obligated to provide such coverage to the extent
that the cost of such coverage exceeds 150% of the cost immediately prior to the
effective time of the merger.

  Affiliates.

     Anesta agreed to take all action reasonably necessary to cause those
parties identified as Anesta affiliates for pooling purposes to execute and
deliver an affiliate agreement prior to the effective time of the merger.

  Listing.

     Cephalon agreed to apply for approval for listing of the Cephalon common
stock to be issued to Anesta stockholders in the merger on the Nasdaq National
Market.

  Employee Matters.

     Cephalon agreed to treat each Anesta employee who continues employment with
the combined company as if that employee had been employed by Cephalon for the
same period that he or she had been employed by Anesta for purposes of the
employee's coverage under Cephalon's benefits plans.

CONDITIONS TO OBLIGATIONS TO EFFECT THE MERGER

  General Conditions.

     The merger agreement contains various conditions to the parties'
obligations to effect the merger, including:

     - the requisite approval of the Anesta stockholders;

     - the expiration or termination of any waiting period applicable to the
       merger under the HSR Act;

     - the absence of any judicial or quasi-judicial action or litigation that
       restrains or prohibits consummation of the merger and the related
       transactions or that prohibits or limits the ownership, operation or
       control by Cephalon, Anesta or any of their respective subsidiaries of
       any part of their respective businesses or assets, or any action taken by
       any governmental entity seeking to prohibit the merger;

     - effectiveness of the registration of which this proxy
       statement/prospectus is a part; and

     - approval of the Nasdaq National Market, upon official notice of issuance,
       of the listing of the Cephalon common stock to be issued in the merger.

  Additional Conditions to Obligations of Cephalon.

     Cephalon's obligations to complete the merger are subject to certain
additional conditions including:

     - the accuracy of the representations and warranties of Anesta as of the
       date of the merger agreement and as of the date of closing, except for
       such inaccuracies which, individually or in the aggregate, would not have
       an Anesta material adverse effect, as defined in the merger agreement;

     - Anesta's performance in all material respects of each of the obligations
       it has agreed, under the merger agreement, to perform prior to the
       closing, and delivery to Cephalon of an officer's certificate to that
       effect;

     - the absence of any Anesta material adverse effect, as defined in the
       merger agreement;

     - Cephalon's receipt of an affiliate agreement from all parties deemed to
       be Anesta's affiliates, as defined under Rule 145 of the Securities Act;
       and

     - Cephalon's receipt of letters from its independent public accountants and
       the independent public accountants of Anesta, each dated as of the
       closing date, stating that such accountants concur with management's
       conclusion that the merger will qualify as a transaction to be accounted
       for in accordance with the pooling of interests method of accounting
       under the requirements of APB No. 16.

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<PAGE>   45

  Additional Conditions to Obligations of Anesta.

     Anesta's obligations to complete the merger are subject to certain
additional conditions including:

     - the accuracy of the representations and warranties of Cephalon and C
       Merger Sub as of the date of the merger agreement and as of the date of
       the closing, except for such inaccuracies which, individually or in the
       aggregate, would not have a Cephalon material adverse effect, as defined
       in the merger agreement;

     - Cephalon's and C Merger Sub's performance in all material respects of
       each of the obligations it has agreed, under the merger agreement, to
       perform prior to the closing, and delivery to Anesta of an officer's
       certificate to that effect;

     - the absence of any Cephalon material adverse effect, as defined in the
       merger agreement; and

     - Anesta's receipt of an opinion from Anesta's legal counsel stating the
       merger as contemplated in the merger agreement will be treated for
       federal income tax purposes as a reorganization within the meaning of
       section 368(a) of the Code.

TERMINATION; TERMINATION FEES AND EXPENSES

     The merger agreement provides that the merger may be abandoned and the
merger agreement may be terminated prior to the merger's effectiveness, in
various ways including:

     - by mutual written consent of Cephalon and Anesta;

     - by either Cephalon or Anesta if approval of Anesta's stockholders has not
       been obtained;

     - by either Cephalon or Anesta if the merger is not consummated on or
       before December 31, 2000, as long as the party requesting such
       termination did not, by its failure to fulfill an obligation under the
       merger agreement, prevent consummation of the merger;

     - by either Cephalon or Anesta if any governmental entity or arbitrator
       with jurisdiction issues a final order, injunction or decree preventing
       consummation of the merger;

     - by either Cephalon or Anesta upon the other entity's breach or failure to
       comply with its obligations under the merger agreement in any material
       respect, or any representation or warranty of the other entity being
       incorrect except as would not result in a material adverse effect, as
       defined in the merger agreement;

     - by Cephalon if the board of directors of Anesta does not recommend the
       merger to Anesta's stockholders or withdraws or modifies its approval or
       recommendation of the merger agreement in any materially adverse manner,
       or recommends to the stockholders of Anesta any acquisition proposal,
       other than by Cephalon; or

     - by Anesta if Anesta determines that an acquisition proposal is a superior
       proposal or has changed its recommendation concerning the merger, and
       Anesta has complied with its obligations regarding non-solicitation under
       the merger agreement.

     If the merger agreement is terminated, it becomes void and none of the
parties to the agreement has any liability or further rights or obligations
under the agreement other than the remedies described below. However, if any
party willfully breaches any of its representations and warranties, or breaches
any of its covenants or agreements under the merger agreement, the breaching
party remains fully liable to the non-breaching parties for such breach.

     If the merger agreement is terminated, each party is responsible for the
expenses it incurs, except that if either party terminates the merger agreement
because the other party has breached, or failed to comply with, any of its
material obligations, or any representation or warranty made by either party is
breached in any material respect except as would not have a material adverse
effect, as defined in the merger agreement, the breaching party will pay up to
$1 million of the non-breaching party's expenses.

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<PAGE>   46

     However, if:

     - Cephalon terminates the agreement because the board of directors of
       Anesta does not recommend the approval of the merger agreement or the
       merger to the stockholders of Anesta or withdraws or modifies such
       approval or recommendation in a manner materially adverse to Cephalon or
       recommends any acquisition proposal, other than by Cephalon;

     - Cephalon terminates the agreement because Anesta breaches its
       non-solicitation obligations under the agreement and such breach is not
       cured within 10 days after notice thereof; or

     - Anesta terminates the agreement because Anesta has determined that an
       acquisition proposal is a superior proposal and has complied with its
       obligations regarding non-solicitation under the merger agreement;

then, if after the date of the merger agreement and prior to such termination,
an acquisition proposal was made, or within one year following such termination,
Anesta enters into an agreement with respect to, or consummates, an acquisition
proposal, Anesta will pay a termination fee of $15,000,000 to Cephalon within
two business days after such termination or entering into such agreement or
consummating such acquisition proposal, plus up to $1,000,000 of Cephalon's
expenses.

     Furthermore, if Anesta terminates the agreement because of a material
breach by Cephalon or C Merger Sub then, Cephalon will pay, within two business
days after such termination or entering into such agreement or consummating such
acquisition proposal, up to $1,000,000 of Anesta's expenses.

AMENDMENT AND WAIVER

     The merger agreement may be amended by Cephalon and Anesta at any time
prior to the effective time. Any amendment must be in writing signed by all the
parties. Once Anesta's stockholders have voted to approve the merger agreement,
however, no party may make an amendment to the merger agreement which would
reduce the amount or change the type of consideration for which each share of
Anesta's common stock will be exchanged or which would materially and adversely
affect Anesta stockholders.

     Prior to the consummation of the merger, any party may:

     - extend the time given for the performance of any of the obligations or
       other acts of the other parties;

     - waive any inaccuracies in the representations and warranties of the other
       parties contained herein or in any document delivered by the other
       parties pursuant to the merger agreement; and

     - waive compliance by the other parties with any of the agreements or
       conditions contained in the merger agreement.

     Any agreement as to extension or waiver by any party will be valid only as
against such party and only if set forth in writing executed by such party.

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<PAGE>   47

                                 THE COMPANIES

CEPHALON

     Cephalon, Inc.,with its principal offices located in West Chester,
Pennsylvania, is a biopharmaceutical company dedicated to the discovery,
development and marketing of products to treat neurological disorders and
cancer.

     Cephalon markets Provigil (modafinil) Tablets [C-IV] for treating excessive
daytime sleepiness associated with narcolepsy. It began marketing the product in
the United States in 1999, and also markets the product in the United Kingdom,
the Republic of Ireland, Italy and Austria. In addition, Cephalon holds rights
to develop and market Provigil in Latin America, South Korea and Taiwan.
Cephalon has an ongoing clinical program to explore the utility of Provigil in
treating excessive sleepiness and fatigue associated with disorders other than
narcolepsy.

     In addition to its clinical program focused on Provigil, Cephalon has other
significant research programs that seek to discover and develop treatments for
neurological disorders and cancer. It has formed alliances with TAP Holdings,
Inc. and Schwarz Pharma AG for the development of signal transduction modulators
to treat cancers, including prostate and pancreatic cancers. Cephalon is also
collaborating with H. Lundbeck A/S for the development of signal transduction
modulators to treat neurodegenerative disorders, including Parkinson's and
Alzheimer's diseases. In addition, Cephalon is collaborating with Leo
Pharmaceuticals for the development of gene transcription regulators for the
treatment of Alzheimer's disease. Cephalon also enters into collaborative
commercial arrangements where it markets the products of third parties,
including Gabitril with Abbott Laboratories.

     Cephalon's research and development efforts focus primarily on two areas:
neurodegenerative disorders and cancers. Neurodegenerative disorders are
characterized by the death of neurons (the specialized conducting cells of the
nervous system) that results in the loss of certain functions such as memory and
motor coordination. Cancers are characterized by the uncontrolled proliferation
of cells that form tumors. Cephalon's research strategy has focused on
understanding the intracellular molecular events that underlie the processes of
cell proliferation, cell survival and cell death. Cephalon utilizes its
technical expertise in molecular biology and chemistry to create novel, orally
active, synthetic molecules to inhibit key targets in intracellular pathways
that govern cell proliferation, survival and death. These novel molecules are
designed to (i) enhance the survival of neurons, thereby intervening in the
progression of neurodegenerative disease or (ii) facilitate the death of tumor
cells leading to new therapies in oncology.

     Cephalon was incorporated in Delaware in 1987. Cephalon's headquarters are
located at 145 Brandywine Parkway, West Chester, PA 19380 and its telephone
number is (610) 344-0200. For further information concerning Cephalon, see
"Cephalon Selected Consolidated Financial Data" and "Where You Can Find More
Information."

C MERGER SUB

     C Merger Sub is a wholly-owned subsidiary of Cephalon. It was incorporated
on January 11, 2000 in the State of Delaware. C Merger Sub has not engaged in
any operations to date and exists solely to facilitate the merger.

ANESTA

     Anesta is a leader in the development of new pharmaceutical products using
its proprietary oral transmucosal system (OTS(TM)) for drug delivery. Anesta's
lead product is Actiq(R) (oral transmucosal fentanyl citrate), which was
approved by the U.S. Food and Drug Administration for marketing in November 1998
and launched in the U.S. in March 1999. Actiq is the first product specifically
designed, studied and approved for breakthrough cancer pain. Actiq is indicated
only for the management of breakthrough cancer pain in patients with
malignancies who are already receiving and who are tolerant to opioid therapy
for their underlying

                                       42
<PAGE>   48

persistent cancer pain. In August 1999, Anesta UK Ltd. submitted a Marketing
Approval Application for Actiq in the United Kingdom. During 1998 and 1999,
Anesta entered into agreements with four European companies to market and
distribute Actiq in the countries of the European Union. Anesta has seven
investigational product candidates in various stages of clinical development;
OTS nicotine for smoking cessation, OTS fentanyl for acute pain management, OTS
etomidate for short-acting sedation, OTS piroxicam for mild to moderate pain,
OTS dorperidol and OTS prochlorperazine for nausea and vomiting and OTS
scopolamine for motion sickness.

     Anesta was incorporated in 1985 under the laws of Utah to commercialize
specific licensed technologies from the University of Utah and reincorporated in
1993 under the laws of Delaware. Anesta has two wholly-owned subsidiaries:
Anesta GmbH located in Horv, Switzerland and Anesta UK Ltd. Located in
Henley-on-Thames, United Kingdom. Anesta is headquartered in Salt Lake City,
Utah.

                                       43
<PAGE>   49

                   UNAUDITED PRO FORMA FINANCIAL INFORMATION

     The following information has been provided to aid you in your analysis of
the financial aspects of the merger. The financial information of Cephalon and
Anesta were derived from the audited consolidated financial statements for the
years ended December 31, 1999, 1998 and 1997 and the unaudited consolidated
financial statements for the six months ended June 30, 2000 and 1999. The
information is only a summary and should be read together with the historical
financial statements and related notes contained in the annual reports and
quarterly reports and other information that we have filed with the Securities
and Exchange Commission and incorporated by reference. The pro forma combined
financial information is not necessarily indicative of the combined financial
positions or results of operations that would have been reported if the
companies had been combined for all periods presented.

POOLING OF INTERESTS ACCOUNTING TREATMENT

     The merger is expected to be accounted for as a "pooling of interests."
This means that, for accounting and financial reporting purposes, the companies
will be treated as if they had always been combined. We have presented unaudited
pro forma financial information that reflects the pooling of interests method of
accounting to provide a picture of what the businesses might have looked like
had they always been combined. The unaudited pro forma statements of operations
and pro forma balance sheets were prepared by combining the historical amounts
of each company. The companies may have performed differently had they always
been combined. You should not rely on the unaudited pro forma financial
information as being indicative of the historical results that would have
occurred or the future results that will occur after the merger.

PERIODS COVERED

     The following unaudited pro forma balance sheets as of June 30, 2000 and
December 31, 1999 are presented as if the merger had occurred on June 30, 2000
and December 31, 1999, respectively. The unaudited pro forma statements of
operations for the six months ended June 30, 2000 and 1999, and for the years
ended December 31, 1999, 1998 and 1997, are presented as if the companies had
always been merged.

                                       44
<PAGE>   50

              UNAUDITED PRO FORMA COMBINED STATEMENT OF OPERATIONS

                         SIX MONTHS ENDED JUNE 30, 2000
              (AMOUNTS IN THOUSANDS, EXCEPT PER SHARE INFORMATION)

<TABLE>
<CAPTION>
                                                                  HISTORICAL
                                                              -------------------   PRO FORMA
                                                              CEPHALON    ANESTA    COMBINED
                                                              --------   --------   ---------
<S>                                                           <C>        <C>        <C>
Revenues:
  Product sales.............................................  $ 28,750   $  3,993   $ 32,743
  License and contract......................................     9,256        678      9,934
                                                              --------   --------   --------
                                                                38,006      4,671     42,677
                                                              --------   --------   --------
Operating expenses:
  Cost of product sales.....................................     5,074      1,023      6,097
  Research and development..................................    27,377      5,165     32,542
  Selling, general and administrative.......................    28,716      9,702     38,418
                                                              --------   --------   --------
                                                                61,167     15,890     77,057
                                                              --------   --------   --------
          Loss from operations..............................   (23,161)   (11,219)   (34,380)
Other income................................................     6,761      1,585      8,346
                                                              --------   --------   --------
          Loss before provision for income taxes............   (16,400)    (9,634)   (26,034)
Provision for income taxes..................................        --        (31)       (31)
                                                              --------   --------   --------
          Net loss..........................................   (16,400)    (9,665)   (26,065)
Dividends on preferred stock................................    (4,531)        --     (4,531)
                                                              --------   --------   --------
          Net loss applicable to common shares..............  $(20,931)  $ (9,665)  $(30,596)
                                                              ========   ========   ========
Basic and diluted loss per share............................  $  (0.63)  $  (0.72)  $  (0.77)
                                                              ========   ========   ========
Weighted average number of shares outstanding...............    33,164     13,373     39,536
                                                              ========   ========   ========
</TABLE>

 See accompanying notes to unaudited pro forma combined financial information.

                                       45
<PAGE>   51

              UNAUDITED PRO FORMA COMBINED STATEMENT OF OPERATIONS

                         SIX MONTHS ENDED JUNE 30, 1999
              (AMOUNTS IN THOUSANDS, EXCEPT PER SHARE INFORMATION)

<TABLE>
<CAPTION>
                                                                  HISTORICAL
                                                              ------------------   PRO FORMA
                                                              CEPHALON   ANESTA    COMBINED
                                                              --------   -------   ---------
<S>                                                           <C>        <C>       <C>
Revenues:
  Product sales.............................................  $  7,252   $ 1,128   $  8,380
  License and contract......................................     7,727       651      8,378
                                                              --------   -------   --------
                                                                14,979     1,779     16,758
                                                              --------   -------   --------
Operating expenses:
  Cost of product sales.....................................       839       371      1,210
  Research and development..................................    20,152     4,835     24,987
  Selling, general and administrative.......................    23,780     4,062     27,842
                                                              --------   -------   --------
                                                                44,771     9,268     54,039
                                                              --------   -------   --------
          Loss from operations..............................   (29,792)   (7,489)   (37,281)
Other income (expense)......................................    (1,650)    2,049        399
                                                              --------   -------   --------
          Loss before provision for income taxes............   (31,442)   (5,440)   (36,882)
Provision for income taxes..................................        --       (15)       (15)
                                                              --------   -------   --------
          Net loss..........................................  $(31,442)  $(5,455)  $(36,897)
                                                              ========   =======   ========
Basic and diluted loss per share............................  $  (1.09)  $ (0.41)  $  (1.05)
                                                              ========   =======   ========
Weighted average number of shares outstanding...............    28,880    13,171     35,156
                                                              ========   =======   ========
</TABLE>

 See accompanying notes to unaudited pro forma combined financial information.

                                       46
<PAGE>   52

              UNAUDITED PRO FORMA COMBINED STATEMENT OF OPERATIONS

                          YEAR ENDED DECEMBER 31, 1999
              (AMOUNTS IN THOUSANDS, EXCEPT PER SHARE INFORMATION)

<TABLE>
<CAPTION>
                                                                  HISTORICAL
                                                              -------------------   PRO FORMA
                                                              CEPHALON    ANESTA    COMBINED
                                                              --------   --------   ---------
<S>                                                           <C>        <C>        <C>
Revenues:
  Product sales.............................................  $ 25,370   $  2,232   $ 27,602
  License and contract......................................    19,549      4,283     23,832
                                                              --------   --------   --------
                                                                44,919      6,515     51,434
                                                              --------   --------   --------
Operating expenses:
  Cost of product sales.....................................     3,250        671      3,921
  Research and development..................................    46,420     10,063     56,483
  Selling, general and administrative.......................    50,992      9,141     60,133
                                                              --------   --------   --------
                                                               100,662     19,875    120,537
                                                              --------   --------   --------
          Loss from operations..............................   (55,743)   (13,360)   (69,103)
Other income (expense)......................................    (3,014)     3,892        878
                                                              --------   --------   --------
          Loss before provision for income taxes............   (58,757)    (9,468)   (68,225)
Provision for income taxes..................................        --        (20)       (20)
                                                              --------   --------   --------
          Net loss before extraordinary charge..............   (58,757)    (9,488)   (68,245)
Extraordinary charge for early extinguishment of debt.......   (11,187)        --    (11,187)
                                                              --------   --------   --------
          Net loss..........................................   (69,944)    (9,488)   (79,432)
Dividends on preferred stock................................    (3,398)        --     (3,398)
                                                              --------   --------   --------
          Net loss applicable to common shares..............  $(73,342)  $ (9,488)  $(82,830)
                                                              ========   ========   ========
Basic and diluted loss per common share:
  Loss per common share before extraordinary charge.........  $  (2.10)  $  (0.72)  $  (2.00)
  Extraordinary charge......................................     (0.38)        --      (0.31)
                                                              --------   --------   --------
                                                              $  (2.48)  $  (0.72)  $  (2.31)
                                                              ========   ========   ========
Weighted average number of shares outstanding...............    29,584     13,227     35,887
                                                              ========   ========   ========
</TABLE>

 See accompanying notes to unaudited pro forma combined financial information.

                                       47
<PAGE>   53

              UNAUDITED PRO FORMA COMBINED STATEMENT OF OPERATIONS

                          YEAR ENDED DECEMBER 31, 1998
              (AMOUNTS IN THOUSANDS, EXCEPT PER SHARE INFORMATION)

<TABLE>
<CAPTION>
                                                                  HISTORICAL
                                                              -------------------   PRO FORMA
                                                              CEPHALON    ANESTA    COMBINED
                                                              --------   --------   ---------
<S>                                                           <C>        <C>        <C>
Revenues:
  Product sales.............................................  $    728   $    193   $    921
  License and contract......................................    14,927        482     15,409
                                                              --------   --------   --------
                                                                15,655        675     16,330
                                                              --------   --------   --------
Operating expenses:
  Cost of product sales.....................................        --         54         54
  Research and development..................................    43,649      8,812     52,461
  Selling, general and administrative.......................    30,947      8,700     39,647
                                                              --------   --------   --------
                                                                74,596     17,566     92,162
                                                              --------   --------   --------
          Loss from operations..............................   (58,941)   (16,891)   (75,832)
Other income................................................     3,534      1,190      4,724
                                                              --------   --------   --------
          Loss before provision for income taxes............   (55,407)   (15,701)   (71,108)
Provision for income taxes..................................        --        (16)       (16)
                                                              --------   --------   --------
          Net loss..........................................  $(55,407)  $(15,717)  $(71,124)
                                                              ========   ========   ========
Basic and diluted loss per share............................  $  (1.95)  $  (1.59)  $  (2.15)
                                                              ========   ========   ========
Weighted average number of shares outstanding...............    28,413      9,898     33,129
                                                              ========   ========   ========
</TABLE>

 See accompanying notes to unaudited pro forma combined financial information.

                                       48
<PAGE>   54

              UNAUDITED PRO FORMA COMBINED STATEMENT OF OPERATIONS

                          YEAR ENDED DECEMBER 31, 1997
              (AMOUNTS IN THOUSANDS, EXCEPT PER SHARE INFORMATION)

<TABLE>
<CAPTION>
                                                                  HISTORICAL
                                                              -------------------   PRO FORMA
                                                              CEPHALON    ANESTA    COMBINED
                                                              --------   --------   ---------
<S>                                                           <C>        <C>        <C>
Revenues:
  Product sales.............................................  $     --   $    184   $    184
  License and contract......................................    23,140          5     23,145
                                                              --------   --------   --------
                                                                23,140        189     23,329
                                                              --------   --------   --------
Operating expenses:
  Cost of product sales.....................................        --         52         52
  Research and development..................................    51,587      8,064     59,651
  Selling, general and administrative.......................    36,744      6,465     43,209
                                                              --------   --------   --------
                                                                88,331     14,581    102,912
                                                              --------   --------   --------
          Loss from operations..............................   (65,191)   (14,392)   (79,583)
Other income................................................     4,772      1,845      6,617
                                                              --------   --------   --------
          Loss before provision for income taxes............   (60,419)   (12,547)   (72,966)
Provision for income taxes..................................        --         (2)        (2)
                                                              --------   --------   --------
          Net loss..........................................  $(60,419)  $(12,549)  $(72,968)
                                                              ========   ========   ========
Basic and diluted loss per share............................  $  (2.36)  $  (1.32)  $  (2.42)
                                                              ========   ========   ========
Weighted average number of shares outstanding...............    25,638      9,500     30,165
                                                              ========   ========   ========
</TABLE>

 See accompanying notes to unaudited pro forma combined financial information.

                                       49
<PAGE>   55

                   UNAUDITED PRO FORMA COMBINED BALANCE SHEET

                                 JUNE 30, 2000
                             (AMOUNTS IN THOUSANDS)

<TABLE>
<CAPTION>
                                                       HISTORICAL
                                                  --------------------    PRO FORMA    PRO FORMA
                                                  CEPHALON     ANESTA    ADJUSTMENTS   COMBINED
                                                  ---------   --------   -----------   ---------
                                                                          (NOTE 2)
<S>                                               <C>         <C>        <C>           <C>
                     ASSETS
Current assets:
  Cash and cash equivalents.....................  $  29,010   $ 16,592     $    --     $  45,602
  Short-term investments........................    121,377     23,078          --       144,455
  Receivables, net..............................     11,017      2,329          --        13,346
  Inventory.....................................     14,465         --          --        14,465
  Other.........................................      1,026      1,293          --         2,319
                                                  ---------   --------     -------     ---------
          Total current assets..................    176,895     43,292          --       220,187
Property and equipment, net.....................     24,339      2,322          --        26,661
Other...........................................      2,453     25,323          --        27,776
                                                  ---------   --------     -------     ---------
                                                  $ 203,687   $ 70,937     $    --     $ 274,624
                                                  =========   ========     =======     =========

      LIABILITIES AND STOCKHOLDERS' EQUITY
Current liabilities:
  Accounts payable..............................  $   7,365   $  1,559     $    --     $   8,924
  Accrued expenses..............................     19,988      1,776       9,000        30,764
  Current portion of unearned revenues..........         --         93          --            93
  Current portion of long-term debt.............      1,977        333          --         2,310
                                                  ---------   --------     -------     ---------
          Total current liabilities.............     29,330      3,761       9,000        42,091
Unearned revenues...............................         --      1,833          --         1,833
Long-term debt..................................     13,486      1,667          --        15,153
Other...........................................        198         --          --           198
                                                  ---------   --------     -------     ---------
          Total liabilities.....................     43,014      7,261       9,000        59,275
                                                  ---------   --------     -------     ---------
Stockholders' equity:
  Preferred stock, $.01 par value, 5,000 shares,
     authorized, 2,500 shares issued and
     outstanding................................         25         --          --            25
  Common stock, $.01 par value, 100,000 shares
     authorized; 41,196 issued and outstanding,
     pro forma..................................        348         14          50           412
  Additional paid in capital....................    529,030    131,608         (50)      660,588
  Treasury stock................................     (1,480)        --          --        (1,480)
  Accumulated deficit...........................   (368,066)   (67,832)     (9,000)     (444,898)
  Accumulated other comprehensive income
     (loss).....................................        816       (114)         --           702
                                                  ---------   --------     -------     ---------
          Total stockholders' equity............    160,673     63,676      (9,000)      215,349
                                                  ---------   --------     -------     ---------
                                                  $ 203,687   $ 70,937     $    --     $ 274,624
                                                  =========   ========     =======     =========
</TABLE>

 See accompanying notes to unaudited pro forma combined financial information.

                                       50
<PAGE>   56

                   UNAUDITED PRO FORMA COMBINED BALANCE SHEET

                               DECEMBER 31, 1999
                             (AMOUNTS IN THOUSANDS)

<TABLE>
<CAPTION>
                                                       HISTORICAL
                                                  --------------------    PRO FORMA    PRO FORMA
                                                  CEPHALON     ANESTA    ADJUSTMENTS   COMBINED
                                                  ---------   --------   -----------   ---------
                                                                          (NOTE 2)
<S>                                               <C>         <C>        <C>           <C>
                     ASSETS
Current assets:
  Cash and cash equivalents.....................  $  13,152   $ 11,746     $    --     $  24,898
  Short-term investments........................    188,410     59,032          --       247,442
  Receivables, net..............................      5,578      1,956          --         7,534
  Inventory.....................................      4,258         --          --         4,258
  Other.........................................        988      1,008          --         1,996
                                                  ---------   --------     -------     ---------
          Total current assets..................    212,386     73,742          --       286,128
Property and equipment, net.....................     20,001      2,466          --        22,467
Other...........................................      1,666      2,001          --         3,667
                                                  ---------   --------     -------     ---------
                                                  $ 234,053   $ 78,209     $    --     $ 312,262
                                                  =========   ========     =======     =========

      LIABILITIES AND STOCKHOLDERS' EQUITY
Current liabilities:
  Accounts payable..............................  $   6,221   $    410     $    --     $   6,631
  Accrued expenses..............................     19,328        795       9,000        29,123
  Current portion of unearned revenues..........         --        746          --           746
  Current portion of long-term debt.............     31,906        333          --        32,239
                                                  ---------   --------     -------     ---------
          Total current liabilities.............     57,455      2,284       9,000        68,739
Unearned revenues...............................         --      1,832          --         1,832
Long-term debt..................................     14,034      1,667          --        15,701
Other...........................................      4,207         --          --         4,207
                                                  ---------   --------     -------     ---------
          Total liabilities.....................     75,696      5,783       9,000        90,479
Stockholders' equity:
  Preferred stock, $.01 par value, 5,000 shares,
     authorized, 2,500 shares issued and
     outstanding................................         25         --          --            25
  Common stock, $.01 par value, 100,000 shares
     authorized; 38,904 issued and outstanding,
     pro forma..................................        326         13          50           389
  Additional paid in capital....................    505,702    130,743         (50)      636,395
  Treasury stock................................     (1,290)        --                    (1,290)
  Accumulated deficit...........................   (347,135)   (58,167)     (9,000)     (414,302)
  Accumulated other comprehensive income
     (loss).....................................        729       (163)         --           566
                                                  ---------   --------     -------     ---------
          Total stockholders' equity............    158,357     72,426      (9,000)      221,783
                                                  ---------   --------     -------     ---------
                                                  $ 234,053   $ 78,209     $    --     $ 312,262
                                                  =========   ========     =======     =========
</TABLE>

  See accompanying notes to unaudited pro forma combined financial information

                                       51
<PAGE>   57

          NOTES TO UNAUDITED PRO FORMA COMBINED FINANCIAL INFORMATION

(1) The unaudited pro forma combined financial statements for Cephalon and
    Anesta give retroactive effect to the proposed merger, which will be
    accounted for as a pooling of interests and, as a result, such statements
    are presented as if the companies had been combined for all periods
    presented. There were no material differences between the accounting
    policies of Cephalon and Anesta. Certain amounts have been reclassified to
    conform the pro forma presentation.

(2) Transaction costs will be incurred to complete the merger and consist
    primarily of financial advisor, legal, accounting and consulting fees, and
    printing, mailing, and registration expenses. Due to the non-recurring
    nature of these costs, they have not been reflected in the pro forma
    condensed combined statements of operations. These expenses will be included
    in the results of operations in the quarter the merger is completed. The pro
    forma combined balance sheets include an accrual of $9.0 million in
    estimated transaction costs.

(3) Pro forma basic and diluted loss per share have been computed using the pro
    forma weighted average number of shares of common stock outstanding during
    the periods. Pro forma basic and diluted net loss per share are the same
    since common stock equivalents outstanding are antidilutive for all periods
    presented. As a result of the merger, each outstanding share of Anesta
    common stock will be converted into the right to receive 0.4765 shares of
    Cephalon common stock.

                                       52
<PAGE>   58

                    MARKET PRICE DATA AND DIVIDEND POLICIES

     Cephalon common stock and Anesta common stock are included in the National
Association of Securities Dealers Automated Quotations System under the symbols
"CEPH" and "NSTA," respectively. These tables set forth, for the periods
indicated, the range of high and low per share closing sales prices for Cephalon
common stock and Anesta common stock as reported on the Nasdaq National Market.

<TABLE>
<CAPTION>
                                                                 CEPHALON
                                                               COMMON STOCK
                                                              ---------------
                                                               LOW      HIGH
                                                              ------   ------
<S>                                                           <C>      <C>
FISCAL YEAR 1998
First quarter...............................................  $ 9.81   $14.63
Second quarter..............................................    6.94    15.38
Third quarter...............................................    4.00     8.06
Fourth quarter..............................................    4.69     9.19
FISCAL YEAR 1999
First quarter...............................................    7.50    10.69
Second quarter..............................................    9.38    17.38
Third quarter...............................................   16.06    21.94
Fourth quarter..............................................   14.00    36.75
FISCAL YEAR 2000
First quarter...............................................   32.63    72.31
Second quarter..............................................   36.19    65.00
Third quarter (through September 5, 2000)...................   39.25    80.00
</TABLE>

<TABLE>
<CAPTION>
                                                                  ANESTA
                                                               COMMON STOCK
                                                              ---------------
                                                               LOW      HIGH
                                                              ------   ------
<S>                                                           <C>      <C>
FISCAL YEAR 1998
First quarter...............................................  $16.00   $20.75
Second quarter..............................................   14.00    20.00
Third quarter...............................................   11.81    18.63
Fourth quarter..............................................   13.88    27.25
FISCAL YEAR 1999
First quarter...............................................   18.25    28.44
Second quarter..............................................   12.56    21.38
Third quarter...............................................    8.06    19.06
Fourth quarter..............................................    8.38    18.31
FISCAL YEAR 2000
First quarter...............................................   13.00    26.75
Second quarter..............................................   12.69    24.88
Third quarter (through September 5, 2000)...................   19.00    32.63
</TABLE>

     Neither Cephalon nor Anesta has ever declared or paid cash dividends on its
common stock. The policies of Cephalon and Anesta are to retain cash, cash
equivalents and investments for use in their respective businesses.

     Following the merger, Cephalon common stock will continue to be listed on
Nasdaq, and there will be no further market for Anesta common stock.

                                       53
<PAGE>   59

                        MANAGEMENT AND OTHER INFORMATION

     After the merger, Anesta will be a wholly-owned subsidiary of Cephalon, and
all of Anesta's subsidiaries will be indirect wholly-owned subsidiaries of
Cephalon. Information relating to the management, executive compensation, the
security ownership of certain beneficial owners and management, certain
relationships and related transactions and other related matters pertaining to
Cephalon and Anesta is contained in or incorporated by reference in their
respective annual reports on Form 10-K, which are incorporated in this proxy
statement/prospectus. See "Where You Can Find More Information."

                   COMPARISON OF THE RIGHTS OF THE HOLDERS OF
                 CEPHALON COMMON STOCK AND ANESTA COMMON STOCK

     The rights of both Cephalon and Anesta stockholders are governed by the
Delaware General Corporation Law, and the respective certificates of
incorporation and bylaws of Cephalon and Anesta. Upon completion of the merger,
Anesta stockholders will become Cephalon stockholders. No changes to the
Cephalon certificate of incorporation or bylaws will be adopted in connection
with the merger.

     The material differences between the rights of Cephalon stockholders and
Anesta stockholders, resulting from differences in their respective certificates
of incorporation and bylaws, are summarized below. This summary is not a
complete discussion of, and is qualified by reference to, Delaware law,
including the Delaware General Corporation Law, the common and constitutional
law of Delaware, and the full texts of the certificate of incorporation and
bylaws of Cephalon and Anesta.

STOCKHOLDER ACTION BY CONSENT

     Under the Delaware General Corporation Law, unless a corporation's
certificate of incorporation provides otherwise, any action required or
permitted to be taken at a stockholders' meeting may be taken without a meeting,
without prior notice and without a vote if a written consent, setting forth the
action so taken, is signed by the holders of outstanding stock having not less
than the minimum number of votes that would be necessary to authorize or take
such action at a meeting at which all shares entitled to vote upon such action
were present and voted.

     Cephalon's bylaws provide that stockholders may consent in writing to any
action required or permitted to be taken at a meeting without such a meeting.

     Anesta's certificate of incorporation expressly provides that no action
required or permitted to be taken at a common stockholders' meeting may be taken
without a meeting. Anesta's certificate of incorporation also provides that the
power of common stockholders to consent in writing is specifically denied.

SPECIAL STOCKHOLDERS' MEETINGS

     The Delaware General Corporation Law provides that a special stockholders'
meeting may be called by the corporation's board of directors or by such person
or persons as the certificate of incorporation or the bylaws may authorize.

     Cephalon's bylaws provide that a special meeting may be called by:

     - the chairman of the board;

     - the majority of the board of directors;

     - the president; or

     - upon written request of the holders of the majority of the outstanding
       common stock.

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<PAGE>   60

     Anesta's certificate of incorporation provides that the following persons
may call a special meeting:

     - the chairman of the board of directors;

     - the chief executive officer; or

     - the board of directors pursuant to a resolution approved by a majority of
       the members of the board then in office.

     Anesta's certificate of incorporation does not permit the stockholders to
call a special meeting.

ADVANCE NOTICE OF DIRECTOR NOMINATIONS AND OTHER STOCKHOLDER PROPOSALS

     Cephalon's bylaws provide that in order to nominate directors or bring
business before an annual meeting, stockholders must provide written notice to
the secretary of Cephalon at least 70 days before the date that is the
anniversary of the prior year's annual meeting but no earlier than 90 days
before such date. However, if the date of the annual meeting is more than 20
days before or 70 days after the anniversary date of the prior year's annual
meeting, notice must be delivered at least 70 days before the meeting date or
within ten days of the date of the public announcement of the meeting but no
earlier than 90 days before the date of the meeting. Cephalon's bylaws contain
similar advance notice requirements for stockholder director nominations at
special meetings. Cephalon's bylaws state that business, other than the
nomination of directors, may only be brought before a special meeting of
stockholders pursuant to the notice of the special meeting.

     Anesta's bylaws provide that in order to nominate directors or bring
business before an annual meeting, stockholders must provide written notice to
the secretary of Anesta at least 60 days before the date that is the anniversary
of the prior year's annual meeting but no earlier than 90 days before such date.
However, if Anesta did not hold a meeting in the previous year or has changed
the annual meeting date by more than 30 days, notice must be delivered at least
60 days before the meeting date or, in the event that the public announcement of
the meeting is first made by Anesta with fewer than 70 days remaining before the
meeting date, within ten days of the date of the public announcement of the
meeting, but no earlier than 90 days before the date of the meeting. Anesta's
bylaws contain similar advance notice requirements for stockholder director
nominations at special meetings. Anesta's bylaws state that business, other than
the nomination of directors, may only be brought before a special meeting of
stockholders pursuant to the notice of the special meeting.

NUMBER AND ELECTION OF DIRECTORS

     The Delaware General Corporation Law permits a corporation's certificate of
incorporation or bylaws to contain provisions governing the number and terms of
directors. However, if the certificate of incorporation contains provisions
fixing the number of directors, such number may be changed only by an amendment
to the certificate of incorporation.

     Cephalon's bylaws provide that the number of directors may not be less than
three nor more than seven. Cephalon's board can change these minimum and maximum
numbers of directors through amendment of the bylaws. The number of directors is
set through the adoption of a resolution of the board of directors. Cephalon
currently has a seven-member board. Cephalon's bylaws also provide that
directors will hold office until the next annual meeting of stockholders.

     Anesta's certificate of incorporation provides that the number of directors
shall be fixed by a resolution adopted by a majority of the board of directors
and that its directors will hold office until the next annual meeting of
stockholders. Anesta currently has a seven-member board.

AMENDMENT OF BYLAWS

     Under Delaware corporate law, bylaws may be adopted, amended or repealed by
the stockholders entitled to vote thereon; provided, however, that any
corporation may, in its certificate of incorporation,

                                       55
<PAGE>   61

confer this power upon the directors, provided the power vested in the
stockholders shall not be divested or limited where the board of directors also
has such power.

     The Cephalon certificate of incorporation provides that the board of
directors may adopt, amend or repeal the bylaws. The Cephalon bylaws provide
that the bylaws may also be adopted, amended or repealed upon the vote of a
majority of the Cephalon voting stock.

     The Anesta certificate of incorporation provides that bylaws may be
adopted, amended or repealed by the board of directors or by the vote of at
least two-thirds of the Anesta voting stock.

REMOVAL OF DIRECTORS

     The Delaware General Corporation Law provides that a director or directors
may be removed with or without cause by the holders of a majority of the shares
then entitled to vote at an election of directors, with certain exceptions.

     Neither Cephalon's bylaws nor its certificate of incorporation provide for
removal of directors.

     Anesta's certificate of incorporation provides that, subject to the rights
of holders of preferred stock, a director may be removed only for cause by an
affirmative vote of stockholders holding a majority of the outstanding shares
stock entitled to vote at an election of directors.

VACANCIES

     Under the Delaware General Corporation Law, unless the corporation's
certificate of incorporation or bylaws provides otherwise, vacancies on the
board of directors and newly created directorships resulting from an increase in
the authorized number of directors may be filled by:

     - a majority of the directors then in office, although less than a quorum;
       or

     - by the sole remaining director.

     If, immediately prior to the filling of any such vacancy or newly created
directorship, the directors in office constitute less than a majority of the
whole board, the Delaware Court of Chancery may, upon application of any
stockholder or stockholders holding at least 10% of the total number of
outstanding shares entitled to vote for such directors, summarily order an
election to fill any such vacancy or newly created directorship, or replace the
directors chosen by the directors then in office.

     Cephalon's bylaws provide that vacancies and newly created directorships
resulting from an increase in the number of directors may be filled by the
holders of a majority of the directors then in office, even though the number of
remaining directors in office is less than a quorum, or by the sole remaining
director.

     Anesta's certificate of incorporation provides that vacancies and newly
created directorships resulting from an increase in the number of directors may
be filled by the vote of a majority of the directors in office at the time of
the vote. Anesta's certificate of incorporation expressly states that such
vacancies and newly created directorships may not be filled by stockholders.

PREFERRED STOCK

     The certificate of incorporation of Cephalon and the certificate of
incorporation of Anesta authorize the respective boards of directors to issue
shares of preferred stock in one or more series and to fix the designations,
preferences, powers and rights of the shares to be included in each series.

     The Cephalon certificate of incorporation reserves 5,000,000 shares of the
total authorized capital stock of Cephalon for issuance as preferred stock. Of
this 5,000,000 reserve, 2,000,000 shares of convertible exchangeable preferred
stock have been issued and 1,000,000 shares have been reserved for issuance in
connection with Cephalon's stockholder rights plan.

                                       56
<PAGE>   62

     The Anesta certificate of incorporation reserves 1,000,000 shares of the
total authorized capital stock of Anesta for issuance as preferred stock.

RIGHTS PLAN

     In November 1993, the Cephalon board of directors adopted a stockholder
rights plan and declared a dividend of one preferred share purchase right for
each outstanding share of Cephalon common stock. In addition a right attaches to
and is transferred with each new issue of Cephalon common stock. The purpose of
the plan is to give the Cephalon board sufficient time to respond, consistent
with its fiduciary duties, to a takeover attempt. Each right entitles the
holder, upon the occurrence of certain events, including the acquisition of 20%
or more of the outstanding Cephalon common stock and the commencement of a
tender offer for 20% or more of the outstanding Cephalon common stock, to
purchase from Cephalon a unit consisting of one one-hundredth of a share of the
Series A Junior Participating Preferred Stock of Cephalon, or a combination of
securities and assets of equivalent value, at a purchase price of $200.00 per
unit (subject to further adjustment). The rights are redeemable under certain
circumstances. The rights under the plan are not triggered by the transactions
contemplated by the merger agreement.

     Anesta does not currently have any agreement or plan in effect that is
similar to the Cephalon Rights Plan.

                            INDEPENDENT ACCOUNTANTS

     It is expected that representatives of PricewaterhouseCoopers LLP will be
present at the special meeting of Anesta stockholders to respond to appropriate
questions from stockholders and to make a statement if they so desire.

                                 LEGAL MATTERS

     The validity of the Cephalon common stock to be issued in the merger has
been passed upon for Cephalon by Morgan, Lewis & Bockius LLP. Certain tax
consequences of the merger have been passed upon for Anesta by Cooley Godward
LLP and for Cephalon by Morgan, Lewis & Bockius LLP.

     David R. King, a partner at Morgan, Lewis & Bockius LLP, is a director of
Cephalon. As of August 31, 2000, Mr. King held options to acquire 25,000 shares
of Cephalon stock, of which 3,750 options are currently exercisable or will be
exercisable within 60 days from such date.

                                    EXPERTS

     The consolidated financial statements of Cephalon, Inc. included in the
Annual Report on Form 10-K for the year ended December 31, 1999 of Cephalon,
Inc. incorporated by reference in this proxy statement/prospectus and elsewhere
in the registration statement have been audited by Arthur Andersen LLP,
independent public accountants, as indicated in their report with respect
thereto, and are incorporated by reference herein in reliance upon the authority
of said firm as experts in giving said reports.

     The financial statements of Anesta incorporated in this proxy
statement/prospectus by reference to the Annual Report on Form 10-K for the year
ended December 31, 1999, have been so incorporated in reliance on the report of
PricewaterhouseCoopers LLP, independent accountants, given on the authority of
said firm as experts in auditing and accounting.

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<PAGE>   63

                      WHERE YOU CAN FIND MORE INFORMATION

     Cephalon and Anesta each file annual, quarterly and current reports, proxy
statements and other information with the SEC. You may read and copy any
reports, statements or other information that the companies file at the SEC's
public reference facilities at 450 Fifth Street, N.W., Washington, D.C. 20549,
and at the following Regional offices of the SEC: Northeast Regional Office, 7
World Trade Center, Suite 1300, New York, New York 10048 and Midwest Regional
Office, Citicorp Center, Suite 1400, 500 W. Madison Street, Chicago, Illinois
60661. Please call the SEC at 1-800-SEC-0330 for further information on the
public reference rooms. Cephalon's and Anesta's public filings are also
available to the public from commercial document retrieval services and at the
Internet web site maintained by the SEC at http://www.sec.gov. Reports, proxy
statements and other information concerning Cephalon and Anesta also may be
inspected at the offices of the National Association of Securities Dealers,
Inc., Listing Section, 1735 K Street, Washington, D.C. 20006.

     Cephalon has filed a Form S-4 registration statement to register with the
SEC the offering and sale of the shares of Cephalon common stock to be issued to
Anesta stockholders in the merger. This proxy statement/prospectus is a part of
that registration statement and constitutes a prospectus of Cephalon and a proxy
statement of Anesta for the special meeting of stockholders.

     As allowed by SEC rules, this proxy statement/prospectus does not contain
all the information that stockholders can find in the registration statement or
the exhibits to the registration statement.

     The SEC allows Cephalon and Anesta to incorporate information in this proxy
statement/prospectus "by reference," which means that the companies can disclose
important information to you by referring you to another document filed
separately with the SEC. The information incorporated by reference is considered
to be part of this proxy statement/prospectus, except for any information
superseded by information contained directly in this proxy statement/prospectus.
This proxy statement/prospectus incorporates by reference the documents listed
below that Cephalon and Anesta have previously filed with the SEC. These
documents contain important information about the companies and their financial
condition.

CEPHALON FILINGS (FILE NO. 000-19119):

     - Annual Report on Form 10-K for fiscal year ended December 31, 1999,
       including all material incorporated by reference therein

     - Quarterly Report on Form 10-Q for fiscal quarter ended March 31, 2000,
       including all material incorporated by reference therein

     - Quarterly Report on Form 10-Q for fiscal quarter ended June 30, 2000,
       including all materials incorporated therein.

     - Current Reports on Form 8-K filed on January 5, 2000, July 21, 2000 and
       July 31, 2000, including all materials incorporated therein

     - Definitive Revised Proxy Statement on Schedule 14A filed on April 17,
       2000, including all material incorporated by reference therein

     - Registration Statement on Form 8-A filed with the SEC on March 15, 1991,
       setting forth the description of the Cephalon common stock, including all
       material incorporated by reference therein

     - Form 8-A/A Registration Statements filed with the SEC on January 20, 1999
       and August 2, 2000, containing the description of the Cephalon
       stockholder rights plan, including all material incorporated by reference
       therein

ANESTA FILINGS (FILE NO. 000-23160):

     - Annual Report on Form 10-K for fiscal year ended December 31, 1999,
       including all material incorporated by reference therein

     - Quarterly Report on Form 10-Q for fiscal quarter ended March 31, 2000,
       including all material incorporated by reference therein

                                       58
<PAGE>   64

     - Amended Quarterly Report on Form 10-Q/A for fiscal quarter ended March
       31, 2000, including all material incorporated by reference therein

     - Quarterly Report on Form 10-Q for fiscal quarter ended June 30, 2000,
       including all material incorporated by reference therein.

     - Current Reports on Form 8-K filed on March 24, 2000, July 21, 2000 and
       August 3, 2000, including all material incorporated by reference therein

     - Definitive Proxy Statement on Schedule 14A filed on May 1, 2000,
       including all material incorporated by reference therein

     Cephalon and Anesta hereby incorporate by reference additional documents
that Cephalon or Anesta may file with the SEC between the date of this proxy
statement/prospectus and the date of the special meeting of Anesta's
stockholders. These, among other things, include periodic reports, such as
annual reports on Form 10-K, quarterly reports on Form 10-Q and current reports
on Form 8-K, as well as proxy statements.

     Cephalon has supplied all information contained or incorporated by
reference in this proxy statement/ prospectus relating to Cephalon or Merger
Sub, and Anesta has supplied all information relating to Anesta.

     If you are a stockholder of Anesta, you may have received some of the
documents incorporated by reference. You may also obtain any of those documents
from the appropriate company or the SEC or the SEC's Internet web site described
above. Documents incorporated by reference are available from the appropriate
company without charge, excluding all exhibits, unless specifically incorporated
by reference as an exhibit in this proxy statement/prospectus. Stockholders may
obtain documents incorporated by reference in this proxy statement/prospectus by
requesting them in writing or by telephone from the appropriate company at the
following addresses:

<TABLE>
<S>                                            <C>
                CEPHALON, INC.                                  ANESTA CORP.
            145 Brandywine Parkway                          4745 Wiley Post Way
            West Chester, PA 19380                        Salt Lake City, UT 84116
             Tel: (610) 344-0200                            Tel: (801) 595-1405
                                                           Email: [email protected]
</TABLE>

     IF YOU WOULD LIKE TO REQUEST DOCUMENTS, PLEASE DO SO BY OCTOBER 3, 2000 TO
RECEIVE THEM BEFORE THE SPECIAL MEETING OF ANESTA STOCKHOLDERS. If you request
any incorporated documents, the appropriate company will mail them to you by
first-class mail, or other equally prompt means, within one business day of
receipt of your request.

     YOU SHOULD RELY ONLY ON THE INFORMATION CONTAINED OR INCORPORATED BY
REFERENCE IN THIS PROXY STATEMENT/PROSPECTUS TO VOTE YOUR SHARES AT THE SPECIAL
MEETING. WE HAVE NOT AUTHORIZED ANYONE TO PROVIDE YOU WITH INFORMATION THAT
DIFFERS FROM THAT CONTAINED IN THIS PROXY STATEMENT/ PROSPECTUS. THIS PROXY
STATEMENT/PROSPECTUS IS DATED SEPTEMBER 8, 2000. YOU SHOULD NOT ASSUME THAT THE
INFORMATION CONTAINED IN THIS PROXY STATEMENT/ PROSPECTUS IS ACCURATE AS OF ANY
DATE OTHER THAN THAT DATE, AND NEITHER THE MAILING OF THIS PROXY
STATEMENT/PROSPECTUS TO STOCKHOLDERS NOR THE ISSUANCE OF SHARES OF CEPHALON
COMMON STOCK IN THE MERGER SHALL CREATE ANY IMPLICATION TO THE CONTRARY.

     Cephalon, Inc. is a registered trademark or trademark of Cephalon, Inc. in
the U.S. and in other select countries. Provigil is a registered trademark of
Genelco, S.A. and is licensed to Cephalon. Gabitril is a registered trademark of
Abbott Laboratories, Inc. and is licensed to Cephalon. Anesta Corp., the Anesta
logos and all other Anesta product and service names are registered trademarks
or trademarks of Anesta Corp. in the U.S. and in other select countries.
"Registered Trademark" and "TM" indicate U.S. registration and U.S. trademark,
respectively. Other third party logos and product/trade names are registered
trademarks or trade names of their respective companies.

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