<PAGE>
                                                                EXHIBIT 4.2



                                  $500,000,000

                                 CEPHALON, INC.

           2 1/2% CONVERTIBLE SUBORDINATED NOTES DUE DECEMBER 15, 2006


                          REGISTRATION RIGHTS AGREEMENT


                                                         December 11, 2001

CREDIT SUISSE FIRST BOSTON CORPORATION,
 As Representative of the Several Purchasers,
   Eleven Madison Avenue
    New York, New York 10010-3629

Ladies and Gentlemen:

Cephalon, Inc., a Delaware corporation (the "COMPANY"), proposes to issue and
sell to Credit Suisse First Boston Corporation (the "LEAD PURCHASER"), Robertson
Stephens, Inc., CIBC World Markets Corp., SG Cowen Securities Corporation, UBS
Warburg LLC, U.S. Bancorp Piper Jaffray Inc., Adams, Harkness & Hill, Inc. and
Banc of America Securities LLC (collectively, the "INITIAL PURCHASERS"), upon
the terms set forth in a purchase agreement dated as of December 5, 2001 (the
"PURCHASE AGREEMENT"), $500,000,000 aggregate principal amount (plus an
additional $100,000,000 principal amount pursuant to an option granted
thereunder) of its 2 1/2% Convertible Subordinated Notes due December 15, 2006
(the "INITIAL SECURITIES"). The Initial Securities will be convertible into
shares of common stock, par value $0.01 per share, of the Company (the "COMMON
STOCK") at the conversion price set forth in the Offering Circular dated
December 5, 2001 (the "OFFERING CIRCULAR"). The Initial Securities will be
issued pursuant to an Indenture, dated as of December 11, 2001 (the
"INDENTURE"), among the Company and State Street Bank and Trust Company, as
trustee (the "TRUSTEE"). As a condition to the Initial Purchasers' obligation to
purchase the Initial Securities under the Purchase Agreement, the Company agrees
with the Initial Purchasers, for the benefit of (i) the Initial Purchasers and
(ii) the holders of the Initial Securities and the Common Stock issuable upon
conversion of the Initial Securities (collectively, the "SECURITIES") from time
to time until such time as such Securities have been sold pursuant to a Shelf
Registration Statement (as defined below) (each of the foregoing a "HOLDER" and
collectively the "HOLDERS"), as follows:

        1. Shelf Registration. (a) The Company shall, at its cost, prepare and,
as promptly as practicable (but in no event more than 90 days after so required
or requested pursuant to this Section 1) file with the Securities and Exchange
Commission (the "COMMISSION") and thereafter use all commercially reasonable
efforts to cause to be declared effective as soon as practicable,


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but not later than 180 days after the latest date of original issuance of the
Initial Securities, a registration statement on Form S-3 (the "SHELF
REGISTRATION STATEMENT") relating to the offer and sale of the Transfer
Restricted Securities (as defined in Section 5 hereof) by the Holders thereof
from time to time in accordance with the methods of distribution set forth in
the Shelf Registration Statement and Rule 415 under the Securities Act of 1933,
as amended (the "SECURITIES ACT") (hereinafter, the "SHELF REGISTRATION");
provided, however, that no Holder (other than an Initial Purchaser) shall be
entitled to have the Securities held by it covered by such Shelf Registration
Statement unless such Holder agrees in writing to be bound by all the provisions
of this Agreement applicable to such Holder.

        (b) The Company shall use all commercially reasonable efforts to keep
the Shelf Registration Statement continuously effective in order to permit the
prospectus included therein (the "PROSPECTUS") to be lawfully delivered by the
Holders of the relevant Securities, for a period of two years (or for such
longer period if extended pursuant to Section 2(h) below) from the date of its
effectiveness or such shorter period that will terminate when all the Securities
covered by the Shelf Registration Statement (i) have been sold pursuant thereto
or (ii) are no longer restricted securities (as defined in Rule 144(k) under the
Securities Act, or any successor rule thereof), assuming for this purpose that
the Holders thereof are not affiliates of the Company (in any such case, such
period being called the "SHELF REGISTRATION PERIOD"). The Company shall be
deemed not to have used all commercially reasonable efforts to keep the Shelf
Registration Statement effective during the requisite period if it voluntarily
takes any action that would result in Holders of Securities covered thereby not
being able to offer and sell such Securities during that period, unless such
action is (i) required by applicable law or (ii) taken by the Company in good
faith and contemplated by Section 2(b)(v) below, and the Company thereafter
complies with the requirements of Section 2(h).

        (c) Notwithstanding any other provisions of this Agreement to the
contrary, the Company shall cause the Shelf Registration Statement and the
Prospectus and any amendment or supplement thereto, as of the effective date of
the Shelf Registration Statement, amendment or supplement, (i) to comply in all
material respects with the applicable requirements of the Securities Act and the
rules and regulations of the Commission and (ii) not to contain any untrue
statement of a material fact or omit to state a material fact required to be
stated therein or necessary in order to make the statements therein, in light of
the circumstances under which they were made, not misleading.

        2. Registration Procedures. In connection with the Shelf Registration
contemplated by Section 1 hereof, the following provisions shall apply:

        (a) The Company shall (i) furnish to the Lead Purchaser, (A) prior to
the filing thereof with the Commission, a copy of the Shelf Registration
Statement and each amendment thereof, (B) within one day following the filing
thereof with the Commission, each supplement, if any, to the prospectus included
in the Shelf Registration Statement, and (C) in the event that an Initial
Purchaser (with respect to any portion of an unsold allotment from the original
offering) is participating in the Shelf Registration Statement, shall use its
best efforts to reflect in the Shelf Registration Statement and each amendment
thereof, when so filed with the Commission, such comments as such Initial
Purchaser reasonably may propose; and (ii) include the names of the


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Holders who propose to sell Securities pursuant to the Shelf Registration
Statement as selling securityholders and who have completed and returned to the
Company the questionnaire included as Annex A to the Offering Circular (a
"Completed Questionnaire").

        (b) The Company shall give written notice to the Initial Purchasers and,
in the case of clauses (ii)-(vi) hereof, the Holders of the Securities from whom
the Company has received a Completed Questionnaire (which notice pursuant to
clauses (iii)-(vi) hereof shall be accompanied by an instruction to suspend the
use of the Prospectus until the requisite changes have been made):

                (i) when the Shelf Registration Statement or any amendment
        thereto has been filed with the Commission;

                (ii) and when the Shelf Registration Statement or any
        post-effective amendment thereto has become effective;

                (iii) of any request by the Commission for amendments or
        supplements to the Shelf Registration Statement or the prospectus
        included therein or for additional information;

                (iv) of the issuance by the Commission of any stop order
        suspending the effectiveness of the Shelf Registration Statement or the
        initiation of any proceedings for that purpose;

                (v) of the receipt by the Company or its legal counsel of any
        notification with respect to the suspension of the qualification of the
        Securities for sale in any jurisdiction or the initiation or threatening
        of any proceeding for such purpose; and

                (vi) of the happening of any event that requires the Company to
        make changes in the Shelf Registration Statement or the Prospectus in
        order that the Shelf Registration Statement or the Prospectus does not
        contain an untrue statement of a material fact nor omit to state a
        material fact required to be stated therein or necessary to make the
        statements therein (in the case of the Prospectus, in light of the
        circumstances under which they were made) not misleading.

        (c) The Company shall make every reasonable effort to obtain the
withdrawal at the earliest possible time, of any order suspending the
effectiveness of the Shelf Registration Statement.

        (d) The Company shall, upon request, furnish to each Holder of
Securities included as a selling securityholder in the Shelf Registration,
without charge, at least one copy of the Shelf Registration Statement and any
post-effective amendment thereto, including financial statements and schedules,
and, if the Holder so requests in writing, all exhibits thereto (including
those, if any, incorporated by reference).


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        (e) The Company shall, during the Shelf Registration Period, deliver to
each Holder of Securities included as a selling securityholder in the Shelf
Registration, without charge, as many copies of the Prospectus (including each
preliminary prospectus) included in the Shelf Registration Statement and any
amendment or supplement thereto as such person may reasonably request. The
Company consents, subject to the provisions of this Agreement, to the use of the
Prospectus or any amendment or supplement thereto by each of the selling Holders
of the Securities in connection with the offering and sale of the Securities
covered by the Prospectus, or any amendment or supplement thereto, included in
the Shelf Registration Statement.

        (f) Prior to any public offering of the Securities pursuant to the Shelf
Registration Statement, the Company shall register or qualify or cooperate with
the Holders of the Securities included therein and their respective counsel in
connection with the registration or qualification of the Securities for offer
and sale under the securities or "blue sky" laws of such states of the United
States as any Holder of the Securities reasonably requests in writing and do any
and all other acts or things necessary or advisable to enable the offer and sale
in such jurisdictions of the Securities covered by such Registration Statement;
provided, however, that the Company shall not be required to (i) qualify
generally to do business in any jurisdiction where it is not then so qualified
or (ii) take any action which would subject it to general service of process or
to taxation in any jurisdiction where it is not then so subject.

        (g) The Company shall cooperate with the Holders of the Securities to
facilitate the timely preparation and delivery of certificates representing the
Securities to be sold pursuant to any Registration Statement free of any
restrictive legends and in such denominations and registered in such names as
the Holders may request, and with respect to the Initial Securities in
accordance with the Indenture, a reasonable period of time prior to sales of the
Securities pursuant to the Shelf Registration Statement.

        (h) Upon the occurrence of any event contemplated by paragraphs (ii)
through (v) of Section 2(b) above during the period for which the Company is
required to maintain an effective Shelf Registration Statement, the Company
shall promptly prepare and file a post-effective amendment to the Shelf
Registration Statement or an amendment or supplement to the Prospectus and any
other required document so that, as thereafter delivered to Holders or
purchasers of the Securities, the Prospectus will not contain an untrue
statement of a material fact or omit to state any material fact required to be
stated therein or necessary to make the statements therein, in light of the
circumstances under which they were made, not misleading. If the Company
notifies the Initial Purchasers and the Holders in accordance with paragraphs
(ii) through (v) of Section 2(b) above to suspend the use of the Prospectus
until the requisite changes to the Prospectus have been made, then the Initial
Purchasers and the Holders shall suspend use of such prospectus, and the period
of effectiveness of the Shelf Registration Statement provided for in Section
1(b) above shall be extended by the number of days from and including the date
of the giving of such notice to and including the date when the Initial
Purchasers and the Holders shall have received such amended or supplemented
prospectus pursuant to this Section 2(h).


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        (i) Not later than the effective date of the Shelf Registration
Statement, the Company will obtain CUSIP numbers for the Initial Securities and
the Common Stock registered under the Shelf Registration Statement (and provide
such CUSIP numbers to the Depository Trust Company), and provide the Trustee
with printed certificates for the Initial Securities, in a form eligible for
deposit with The Depository Trust Company.

        (j) The Company will comply with all rules and regulations of the
Commission to the extent and so long as they are applicable to the Shelf
Registration and will make generally available to its security holders (or
otherwise provide in accordance with Section 11(a) of the Securities Act), an
earnings statement satisfying the provisions of Section 11(a) of the Securities
Act, no later than 45 days after the end of a 12-month period (or 90 days, if
such period is a fiscal year) beginning with the first month of the Company's
first fiscal quarter commencing after the effective date of the Shelf
Registration Statement, which statement shall cover such 12-month period.

        (k) The Company shall cause the Indenture to be qualified under the
Trust Indenture Act of 1939, as amended (the "TRUST INDENTURE ACT"), in a timely
manner and containing such changes, if any, as shall be necessary for such
qualification. In the event that such qualification would require the
appointment of a new trustee under the Indenture, the Company shall appoint a
new trustee thereunder pursuant to the applicable provisions of the Indenture.

        (l) The Company may require each Holder of Securities to be sold
pursuant to the Shelf Registration Statement to furnish to the Company such
information regarding the Holder and the distribution of the Securities as the
Company may from time to time reasonably require for inclusion in the Shelf
Registration Statement, and the Company may exclude from such registration the
Securities of any Holder that unreasonably fails to furnish such information
within a reasonable time after receiving such request.

        (m) The Company shall enter into such customary agreements (including,
if requested, an underwriting agreement in customary form) and take all such
other actions, if any, as any Holder shall reasonably request in order to
facilitate the disposition of the Securities pursuant to the Shelf Registration;
provided, however, that the Company is required to facilitate no more than two
underwritten offerings.

        (n) The Company shall (i) make reasonably available for inspection by
the Holders, any underwriter participating in any disposition pursuant to the
Shelf Registration Statement and any attorney, accountant or other agent
retained by the Holders or any such underwriter, all relevant financial and
other records, pertinent corporate documents and properties of the Company and
(ii) cause the Company's officers, directors, employees, accountants and
auditors to supply all relevant information reasonably requested by the Holders
or any such underwriter, attorney, accountant or agent in connection with the
Shelf Registration Statement, in each case, as shall be reasonably necessary to
enable such persons, to conduct a reasonable investigation within the meaning of
Section 11 of the Securities Act; provided, however, that the foregoing
inspection and information gathering shall be coordinated on behalf of the
Initial Purchasers by you and on behalf of the other parties, by one counsel
designated by and on behalf of such other parties as described in Section 3
hereof.


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        (o) The Company, if requested by any Holder of Securities covered by the
Shelf Registration Statement, shall (i) use all commercially reasonable efforts
to cause (A) its counsel to deliver an opinion and updates thereof relating to
the Securities (in form, scope, and substance which is reasonably satisfactory
to the managing underwriters, if any) addressed to such Holders and the managing
underwriters, if any, thereof, and dated, in the case of the initial opinion,
the effective date of such Shelf Registration Statement, and (B) its independent
public accountants to provide to the selling Holders of the applicable
Securities and any underwriter therefor a comfort letter in customary form and
covering matters of the type customarily covered in comfort letters in
connection with primary underwritten offerings, subject to receipt of
appropriate documentation as contemplated, and only if permitted, by Statement
of Auditing Standards No. 72, and (ii) cause its officers to execute and deliver
all customary documents and certificates and updates thereof requested by any
underwriters of the Securities.

        (p) In the event that any broker-dealer registered under the Exchange
Act shall underwrite any Securities or participate as a member of an
underwriting syndicate or selling group or "assist in the distribution" (within
the meaning of the Conduct Rules (the "RULES") of the National Association of
Securities Dealers, Inc. ("NASD")) thereof, whether as a Holder of such
Securities or as an underwriter, a placement or sales agent or a broker or
dealer in respect thereof, or otherwise, the Company will assist such
broker-dealer in complying with the requirements of such Rules, including,
without limitation, by (i) if such Rules, including Rule 2720, shall so require,
engaging a "qualified independent underwriter" (as defined in Rule 2720) to
participate in the preparation of the Shelf Registration Statement relating to
such Securities, to exercise usual standards of due diligence in respect thereto
and, if any portion of the offering contemplated by such Registration Statement
is an underwritten offering or is made through a placement or sales agent, to
recommend the yield of such Securities, (ii) indemnifying any such qualified
independent underwriter to the extent of the indemnification of underwriters
provided in Section 5 hereof and (iii) providing such information to such
broker-dealer as may be required in order for such broker-dealer to comply with
the requirements of the Rules.

        (q) The Company shall use commercially reasonable efforts to take all
other steps necessary to effect the registration of the Securities covered by a
Registration Statement contemplated hereby.

        (r) The Company may suspend use of the Prospectus for a period not to
exceed an aggregate of 45 days in any 90-day period or an aggregate of 90 days
in any twelve-month period in the event of:

                (i) the issuance by the SEC of a stop order suspending the
        effectiveness of the Shelf Registration Statement or the initiation of
        proceedings with respect to the Shelf Registration Statement under
        Section 8(d) or 8(e) of the Securities Act,

                (ii) the occurrence of any event or the existence of any fact as
        a result of which any Registration Statement shall contain any untrue
        statement of a material fact or omit to state any material fact required
        to be stated therein or necessary to make the statements therein not
        misleading, or any Prospectus shall contain any untrue statement


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        of a material fact or omit to state any material fact required to be
        stated therein or necessary to make the statements therein, in the light
        of the circumstances under which they were made, not misleading, or

                (iii) the occurrence or existence of any pending corporate
        development that, in the discretion of the Company, makes it appropriate
        to suspend the availability of the Shelf Registration Statement and the
        related Prospectus.

        In the event of a suspension pursuant to clause (ii) above, subject to
the next sentence, the Company shall as promptly as practicable prepare and file
a post-effective amendment to such Registration Statement or a supplement to the
related Prospectus or any document incorporated therein by reference or file any
other required document that would be incorporated by reference into such
Registration Statement and Prospectus so that such Registration Statement does
not contain any untrue statement of a material fact or omit to state any
material fact required to be stated therein or necessary to make the statements
therein not misleading, and such Prospectus does not contain any untrue
statement of a material fact or omit to state any material fact required to be
stated therein or necessary to make the statements therein, in the light of the
circumstances under which they were made, not misleading, as thereafter
delivered to the purchasers of the Securities being sold thereunder, and, in the
case of a post-effective amendment to a Registration Statement, subject to the
next sentence, use all commercially reasonable efforts to cause it to be
declared effective as promptly as is reasonably practicable, and give notice to
the Holders that the availability of the Shelf Registration Statement is
suspended and, upon receipt of any such notice, each Holder agrees not to sell
any Securities pursuant to the Registration Statement until such Holder's
receipt of copies of the supplemented or amended Prospectus provided for above,
or until it is advised in writing by the Company that the Prospectus may be
used, and has received copies of any additional or supplemental filings that are
incorporated or deemed incorporated by reference in such Prospectus.

        The Company will use all commercially reasonable efforts to ensure that
the use of the Prospectus may be resumed (x) in the case of clause (i) above, as
promptly as is practicable, (y) in the case of clause (ii) above, as soon as, in
the sole judgment of the Company, public disclosure of such material event would
not be prejudicial to or contrary to the interests of the Company or, if
necessary to avoid unreasonable burden or expense, as soon as reasonably
practicable thereafter and (z) in the case of clause (iii) above, as soon as, in
the discretion of the Company, such suspension is no longer appropriate.

        3. Registration Expenses. (a) All expenses incident to the Company's
performance of and compliance with this Agreement will be borne by the Company,
regardless of whether a Registration Statement is ever filed or becomes
effective, including without limitation;

                (i) all registration and filing fees and expenses;

                (ii) all fees and expenses of compliance with federal securities
        and state "blue sky" or securities laws;


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                (iii) all expenses of printing (including printing certificates
        for the Securities to be issued and printing of Prospectuses), messenger
        and delivery services and telephone;

                (iv) all fees and disbursements of counsel for the Company;

                (v) all application and filing fees in connection with listing
        the Securities on a national securities exchange or automated quotation
        system pursuant to the requirements hereof; and

                (vi) all fees and disbursements of independent certified public
        accountants of the Company (including the expenses of any special audit
        and comfort letters required by or incident to such performance).

The Company will bear its internal expenses (including, without limitation, all
salaries and expenses of its officers and employees performing legal or
accounting duties), the expenses of any annual audit and the fees and expenses
of any person, including special experts, retained by the Company.

        (b) In connection with the Shelf Registration Statement required by this
Agreement, the Company will reimburse the Initial Purchasers and the Holders of
Securities covered by the Shelf Registration Statement, for the reasonable fees
and disbursements of not more than one counsel, designated by the Holders of a
majority in principal amount of the Securities covered by the Shelf Registration
Statement (provided that Holders of Common Stock issued upon the conversion of
the Initial Securities shall be deemed to be Holders of the aggregate principal
amount of Initial Securities from which such Common Stock was converted) to act
as counsel for the Holders in connection therewith.

        4. Indemnification. (a) The Company agrees to indemnify and hold
harmless each Holder and each person, if any, who controls such Holder within
the meaning of the Securities Act or the Securities Exchange Act of 1934, as
amended (the "EXCHANGE ACT") (each Holder, and such controlling persons are
referred to collectively as the "INDEMNIFIED PARTIES") from and against any
losses, claims, damages or liabilities, joint or several, or any actions in
respect thereof (including, but not limited to, any losses, claims, damages,
liabilities or actions relating to purchases and sales of the Securities) to
which each Indemnified Party may become subject under the Securities Act, the
Exchange Act or otherwise, insofar as such losses, claims, damages, liabilities
or actions arise out of or are based upon any untrue statement or alleged untrue
statement of a material fact contained in the Shelf Registration Statement or
prospectus including any document incorporated by reference therein, or in any
amendment or supplement thereto or in any preliminary prospectus relating to the
Shelf Registration, or arise out of, or are based upon, the omission or alleged
omission to state therein a material fact required to be stated therein or
necessary to make the statements therein not misleading, and shall reimburse, as
incurred, the Indemnified Parties for any legal or other expenses reasonably
incurred by them in connection with investigating or defending any such loss,
claim, damage, liability or action in respect thereof; provided, however, that
(i) the Company shall not be liable in any such case to the extent that such
loss, claim, damage or liability arises out of or is based upon any untrue
statement or alleged untrue statement or omission or alleged omission made in
the Shelf


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Registration Statement or prospectus or in any amendment or supplement thereto
or in any preliminary prospectus relating to the Shelf Registration in reliance
upon and in conformity with written information pertaining to such Holder and
furnished to the Company by or on behalf of such Holder specifically for
inclusion therein (which shall include, without limitation, the information
provided to the Company by such Indemnified Party in the Completed
Questionnaire) and (ii) with respect to any untrue statement or omission or
alleged untrue statement or omission made in any preliminary prospectus relating
to the Shelf Registration Statement, the indemnity agreement contained in this
subsection (a) shall not inure to the benefit of any Holder from whom the person
asserting any such losses, claims, damages or liabilities purchased the
Securities concerned, to the extent that a prospectus relating to such
Securities was required to be delivered by such Holder under the Securities Act
in connection with such purchase and any such loss, claim, damage or liability
of such Holder results from the fact that there was not sent or given to such
person, at or prior to the written confirmation of the sale of such Securities
to such person, a copy of the final prospectus if the Company had previously
furnished copies thereof to such Holder; provided further, however, that this
indemnity agreement will be in addition to any liability which the Company may
otherwise have to such Indemnified Party. The Company also shall indemnify
underwriters, their officers and directors and each person who controls such
underwriters within the meaning of the Securities Act or the Exchange Act to the
same extent as provided above with respect to the indemnification of the Holders
of the Securities if requested by such Holders.

        (b) Each Holder, severally and not jointly, will indemnify and hold
harmless the Company, its officers and directors and each person, if any, who
controls the Company within the meaning of the Securities Act or the Exchange
Act from and against any losses, claims, damages or liabilities or any actions
in respect thereof, to which the Company or any such controlling person may
become subject under the Securities Act, the Exchange Act or otherwise, insofar
as such losses, claims, damages, liabilities or actions arise out of or are
based upon any untrue statement or alleged untrue statement of a material fact
contained in the Shelf Registration Statement or prospectus or in any amendment
or supplement thereto or in any preliminary prospectus relating to the Shelf
Registration, or arise out of or are based upon the omission or alleged omission
to state therein a material fact necessary to make the statements therein not
misleading, but in each case only to the extent that the untrue statement or
omission or alleged untrue statement or omission was made in reliance upon and
in conformity with written information pertaining to such Holder and furnished
to the Company by or on behalf of such Holder specifically for inclusion therein
(which shall include, without limitation, the information provided to the
Company by such Indemnified Party in the Completed Questionnaire); and, subject
to the limitation set forth immediately preceding this clause, shall reimburse,
as incurred, the Company for any legal or other expenses reasonably incurred by
the Company or any such controlling person in connection with investigating or
defending any loss, claim, damage, liability or action in respect thereof. This
indemnity agreement will be in addition to any liability which such Holder may
otherwise have to the Company or any of its controlling persons.

        (c) Promptly after receipt by an indemnified party under this Section 4
of notice of the commencement of any action or proceeding (including a
governmental investigation), such indemnified party will, if a claim in respect
thereof is to be made against the indemnifying party under this Section 4,
notify the indemnifying party of the commencement thereof; but the


                                       9
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omission so to notify the indemnifying party will not, in any event, relieve the
indemnifying party from any obligations to any indemnified party other than the
indemnification obligation provided in paragraph (a) or (b) above. In case any
such action is brought against any indemnified party, and it notifies the
indemnifying party of the commencement thereof, the indemnifying party will be
entitled to participate therein and, to the extent that it may wish, jointly
with any other indemnifying party similarly notified, to assume the defense
thereof, with counsel reasonably satisfactory to such indemnified party (who
shall not, except with the consent of the indemnified party, be counsel to the
indemnifying party), and after notice from the indemnifying party to such
indemnified party of its election so to assume the defense thereof the
indemnifying party will not be liable to such indemnified party under this
Section 4 for any legal or other expenses, other than reasonable costs of
investigation, subsequently incurred by such indemnified party in connection
with the defense thereof. No indemnifying party shall, without the prior written
consent of the indemnified party, effect any settlement of any pending or
threatened action in respect of which any indemnified party is or could have
been a party and indemnity could have been sought hereunder by such indemnified
party unless such settlement (i) includes an unconditional release of such
indemnified party from all liability on any claims that are the subject matter
of such action, and (ii) does not include a statement as to or an admission of
fault, culpability or a failure to act by or on behalf of any indemnified party.

        (d) If the indemnification provided for in this Section 4 is unavailable
or insufficient to hold harmless an indemnified party under subsections (a) or
(b) above, then each indemnifying party shall contribute to the amount paid or
payable by such indemnified party as a result of the losses, claims, damages or
liabilities (or actions in respect thereof) referred to in subsection (a) or (b)
above in such proportion as is appropriate to reflect the relative fault of the
indemnifying party or parties on the one hand and the indemnified party on the
other in connection with the statements or omissions that resulted in such
losses, claims, damages or liabilities (or actions in respect thereof) as well
as any other relevant equitable considerations. The relative fault of the
parties shall be determined by reference to, among other things, whether the
untrue or alleged untrue statement of a material fact or the omission or alleged
omission to state a material fact relates to information supplied by the Company
on the one hand or such Holder or such other indemnified party, as the case may
be, on the other, and the parties' relative intent, knowledge, access to
information and opportunity to correct or prevent such statement or omission.
The amount paid by an indemnified party as a result of the losses, claims,
damages or liabilities referred to in the first sentence of this subsection (d)
shall be deemed to include any legal or other expenses reasonably incurred by
such indemnified party in connection with investigating or defending any action
or claim which is the subject of this subsection (d). Notwithstanding any other
provision of this Section 4(d), the Holders shall not be required to contribute
any amount in excess of the amount by which the net proceeds received by such
Holders from the sale of the Securities pursuant to the Shelf Registration
Statement exceeds the amount of damages which such Holders have otherwise been
required to pay by reason of such untrue or alleged untrue statement or omission
or alleged omission. No person guilty of fraudulent misrepresentation (within
the meaning of Section 11(f) of the Securities Act) shall be entitled to
contribution from any person who was not guilty of such fraudulent
misrepresentation. For purposes of this paragraph (d), each person, if any, who
controls such indemnified party within the meaning of the Securities Act or the
Exchange Act shall have the same rights to contribution as such


                                       10
<PAGE>


indemnified party and each person, if any, who controls the Company within the
meaning of the Securities Act or the Exchange Act shall have the same rights to
contribution as the Company.

        (e) The agreements contained in this Section 4 shall survive the sale of
the Securities pursuant to the Shelf Registration Statement and shall remain in
full force and effect, regardless of any termination or cancellation of this
Agreement or any investigation made by or on behalf of any indemnified party.

        5. Additional Interest Under Certain Circumstances. (a) Additional
interest (the "ADDITIONAL INTEREST") with respect to the Initial Securities
shall be assessed as follows if any of the following events occur (each such
event in clauses (i) through (iv) below being herein called a "REGISTRATION
DEFAULT"):

                (i) the Shelf Registration Statement has not been filed with the
        Commission by the 90th day after the first date of original issuance of
        the Initial Securities;

                (ii) the Shelf Registration Statement has not been declared
        effective by the Commission by the 180th day after the first date of
        original issue of the Initial Securities; or

                (iii) after the Shelf Registration Statement has been declared
        effective, such Shelf Registration ceases to be effective or usable in
        connection with resales of the Initial Securities and the Common Stock
        issuable upon conversion of the Initial Securities in accordance with
        and during the periods specified in this Agreement and (A) the Company
        does not cure the Shelf Registration Statement within five business days
        by post-effective amendment or report filed pursuant to the Exchange Act
        or (B) if applicable, the Company does not terminate the suspension
        period described in Section 2(s) above by the 45th or 90th day, as the
        case may be.

Each of the foregoing will constitute a Registration Default whatever the reason
for any such event and whether it is voluntary or involuntary or is beyond the
control of the Company or pursuant to operation of law or as a result of any
action or inaction by the Commission.

        Additional Interest shall accrue on the Initial Securities and the
underlying Common Stock over and above the interest set forth in the title of
the Initial Securities from and including the date on which any such
Registration Default shall occur to but excluding the date on which all such
Registration Defaults have been cured, at a rate of 0.50% per annum, or an
equivalent amount for any Common Stock issued upon conversion of the Initial
Securities (the "ADDITIONAL INTEREST RATE").

        (b) A Registration Default referred to in Section 5(a)(iii) hereof shall
be deemed not to have occurred and be continuing in relation to the Shelf
Registration Statement or the related prospectus if (i) such Registration
Default has occurred solely as a result of (x) the filing of a post-effective
amendment to the Shelf Registration Statement to incorporate annual audited
financial information with respect to the Company where such post-effective
amendment is not yet effective and needs to be declared effective to permit
Holders to use the related prospectus or


                                       11
<PAGE>


(y) other material events, with respect to the Company that would need to be
described in such Shelf Registration Statement or the related prospectus and
(ii) in the case of clause (y), the Company is proceeding promptly and in good
faith to amend or supplement the Shelf Registration Statement and related
prospectus to describe such events as required by paragraph 2(h) hereof;
provided, however, that in any case if such Registration Default occurs for a
continuous period in excess of 30 days, Additional Interest shall be payable in
accordance with the above paragraph from the day such Registration Default
occurs until such Registration Default is cured.

        (c) Any amounts of Additional Interest due pursuant to Section 5(a) will
be payable in cash on the regular interest payment dates with respect to the
Initial Securities. The amount of Additional Interest will be determined by
multiplying the applicable Additional Interest Rate by the principal amount of
the Initial Securities, further multiplied by a fraction, the numerator of which
is the number of days such Additional Interest Rate was applicable during such
period (determined on the basis of a 360-day year comprised of twelve 30-day
months), and the denominator of which is 360.

        (d) "TRANSFER RESTRICTED SECURITIES" means each Security until (i) the
date on which such Security has been effectively registered under the Securities
Act and disposed of in accordance with the Shelf Registration Statement or (ii)
the date on which such Security is distributed to the public pursuant to Rule
144 under the Securities Act or is saleable pursuant to Rule 144(k) under the
Securities Act.

        6. Rules 144 and 144A. The Company shall use its best efforts to file
the reports required to be filed by it under the Securities Act and the Exchange
Act in a timely manner and, if at any time the Company is not required to file
such reports, it will, upon the request of any Holder, make publicly available
other information so long as necessary to permit sales of their securities
pursuant to Rules 144 and 144A. The Company covenants that it will take such
further action as any Holder may reasonably request in writing, all to the
extent required from time to time to enable such Holder to sell Transfer
Restricted Securities without registration under the Securities Act within the
limitation of the exemptions provided by Rules 144 and 144A (including the
requirements of Rule 144A(d)(4)). The Company will provide a copy of this
Agreement to prospective purchasers of Securities identified to the Company by
the Initial Purchasers upon written request. Upon the written request of any
Holder, the Company shall deliver to such Holder a written statement as to
whether it has complied with such requirements. Notwithstanding the foregoing,
nothing in this Section 6 shall be deemed to require the Company to register any
of its securities pursuant to the Exchange Act.

        7. Underwritten Registrations. If any of the Transfer Restricted
Securities covered by the Shelf Registration are to be sold in an underwritten
offering, the investment banker or investment bankers and manager or managers
that will administer the offering ("MANAGING UNDERWRITERS") will be selected by
the holders of a majority in aggregate principal amount of such Transfer
Restricted Securities to be included in such offering (provided that holders of
Common Stock issued upon conversion of the Initial Securities shall not be
deemed holders of Common Stock, but shall be deemed to be holders of the
aggregate principal amount of Initial Securities from which such Common Stock
was converted.


                                       12
<PAGE>


        No person may participate in any underwritten registration hereunder
unless such person (i) agrees to sell such person's Transfer Restricted
Securities on the basis reasonably provided in any underwriting arrangements
approved by the persons entitled hereunder to approve such arrangements and (ii)
completes and executes all questionnaires, powers of attorney, indemnities,
underwriting agreements and other documents reasonably required under the terms
of such underwriting arrangements.

        8. Miscellaneous.

        (a) Remedies. The Company acknowledges and agrees that any failure by
the Company to comply with its obligations under Section 1 hereof may result in
material irreparable injury to the Initial Purchasers or the Holders for which
there is no adequate remedy at law, that it will not be possible to measure
damages for such injuries precisely and that, in the event of any such failure,
the Initial Purchasers or any Holder may obtain such relief as may be required
to specifically enforce the Company's obligations under Sections 1 hereof. The
Company further agrees to waive the defense in any action for specific
performance that a remedy at law would be adequate.

        (b) No Inconsistent Agreements. The Company will not on or after the
date of this Agreement enter into any agreement with respect to its securities
that is inconsistent with the rights granted to the Holders in this Agreement or
otherwise conflicts with the provisions hereof. The rights granted to the
Holders hereunder do not in any way conflict with and are not inconsistent with
the rights granted to the holders of the Company's securities under any
agreement in effect on the date hereof.

        (c) Amendments and Waivers. The provisions of this Agreement may not be
amended, modified or supplemented, and waivers or consents to departures from
the provisions hereof may not be given, except by the Company and the written
consent of the holders of a majority in principal amount of the Securities
affected by such amendment, modification, supplement, waiver or consents
(provided that holders of Common Stock issued upon conversion of Initial
Securities shall not be deemed holders of Common Stock, but shall be deemed to
be holders of the aggregate principal amount of Initial Securities from which
such Common Stock was converted). Without the consent of the Holder of each
Initial Security, however, no modification may change the provisions relating to
the payment of Additional Interest.

        (d) Notices. All notices and other communications provided for or
permitted hereunder shall be made in writing by hand delivery, first-class mail,
facsimile transmission, or air courier which guarantees overnight delivery:

                (1) if to a Holder of the Securities, at the most current
        address given by such Holder to the Company.

                (2) if to the Initial Purchasers;

                        c/o Credit Suisse First Boston Corporation


                                       13
<PAGE>


                            Eleven Madison Avenue
                            New York, NY 10010-3629
                            Fax No.: (212) 325-8278
                            Attention: Transactions Advisory Group

        with a copy to:

                        Shearman & Sterling
                        599 Lexington Avenue
                        New York, NY 10022-6069
                        Fax No.: (212) 848-7179
                        Attn: Michael J. Schiavone, Esq.

                (3)     if to the Company, at its address as follows:

                        Cephalon, Inc.
                        145 Brandywine Parkway
                        West Chester, PA 19380
                        Fax No.: (610) 344-7563
                        Attn: General Counsel

        with a copy to:

                        Morgan, Lewis & Bockius LLP
                        1701 Market Street
                        Philadelphia, PA 19103
                        Fax No.: (215) 963-5299
                        Attn: Richard A. Silfen, Esq.

        All such notices and communications shall be deemed to have been duly
given: at the time delivered by hand, if personally delivered; three business
days after being deposited in the mail, postage prepaid, if mailed; when receipt
is acknowledged by recipient's facsimile machine operator, if sent by facsimile
transmission; and on the day delivered, if sent by overnight air courier
guaranteeing next day delivery.

        (e) Third Party Beneficiaries. The Holders shall be third party
beneficiaries to the agreements made hereunder between the Company, on the one
hand, and the Initial Purchasers, on the other hand, and shall have the right to
enforce such agreements directly to the extent they may deem such enforcement
necessary or advisable to protect their rights or the rights of Holders
hereunder.

        (f) Successors and Assigns. This Agreement shall be binding upon the
Company and its successors and assigns.


                                       14
<PAGE>


        (g) Counterparts. This Agreement may be executed in any number of
counterparts and by the parties hereto in separate counterparts, each of which
when so executed shall be deemed to be an original and all of which taken
together shall constitute one and the same agreement.

        (h) Headings. The headings in this Agreement are for convenience of
reference only and shall not limit or otherwise affect the meaning hereof.

        (i) Governing Law. THIS AGREEMENT SHALL BE GOVERNED BY, AND CONSTRUED IN
ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK WITHOUT REGARD TO PRINCIPLES
OF CONFLICTS OF LAWS.

        By the execution and delivery of this Agreement, the Company submits to
the nonexclusive jurisdiction of any federal or state court in the State of New
York.

        (j) Severability. If any one or more of the provisions contained herein,
or the application thereof in any circumstance, is held invalid, illegal or
unenforceable, the validity, legality and enforceability of any such provision
in every other respect and of the remaining provisions contained herein shall
not be affected or impaired thereby.

        (k) Securities Held by the Company. Whenever the consent or approval of
Holders of a specified percentage of principal amount of Securities is required
hereunder, Securities held by the Company or its affiliates (other than
subsequent Holders of Securities if such subsequent Holders are deemed to be
affiliates solely by reason of their holdings of such Securities) shall not be
counted in determining whether such consent or approval was given by the Holders
of such required percentage.


                                       15
<PAGE>


        If the foregoing is in accordance with your understanding of our
agreement, please sign and return to the Company a counterpart hereof, whereupon
this instrument, along with all counterparts, will become a binding agreement
among the several Initial Purchasers and the Company in accordance with its
terms.

                                 Very truly yours,

                                 Cephalon, Inc.


                                 by        /s/ J. KEVIN BUCHI
                                   ---------------------------------------------
                                        Name:  J. Kevin Buchi
                                        Title: Senior VP and CFO


The foregoing Registration Rights
Agreement is hereby confirmed
and accepted as of the date first
above written.

CREDIT SUISSE FIRST BOSTON CORPORATION
ROBERTSON STEPHENS, INC.
CIBC WORLD MARKETS CORP.
SG COWEN SECURITIES CORPORATION
UBS WARBURG LLC
U.S. BANCORP PIPER JAFFRAY INC.
ADAMS, HARKNESS & Hill, INC.
BANC OF AMERICA SECURITIES LLC

BY: CREDIT SUISSE FIRST BOSTON CORPORATION


        by         /s/ PETE A. MEYERS
          ---------------------------------------------
                Name:  Pete A. Meyers
                Title: Director