Document:

<PAGE>

                                                                 EXHIBIT 10.5(N)

                                              January 27, 2000

Laboratoire L. Lafon
19 Avenue du Professeur-Cadiot
94701 Maisons Alfort
France

        Re:  Amendment No. 7 to License Agreement and Supply Agreement
             ---------------------------------------------------------

Gentlemen:

     This letter agreement shall serve as an amendment to (a) the License
Agreement dated January 20, 1993, as previously amended (the "License
Agreement") between Cephalon, Inc. ("Cephalon") and Laboratoire L. Lafon
("Lafon"), and (b) the Supply Agreement dated January 20, 1993, as previously
amended (the "Supply Agreement") between Cephalon and Lafon.  All capitalized
terms not otherwise defined herein shall be used as defined in the License
Agreement.

     1.  The term "Territory," for all purposes under the License Agreement and
the Supply Agreement, is hereby expanded to include all of the countries of
Central and South America (collectively, the "Latin American Territory").  In
addition, the United Mexican States ("Mexico") hereafter shall be treated as
part of the Latin American Territory and, therefore, to the extent that the
terms of this Amendment modify or conflict with any of the terms and conditions
(including, without limitation, financial terms) previously established under
the License Agreement and the Supply Agreement relating to the sale and
marketing of the Product in Mexico, then such terms are hereby superceded by the
terms of this Amendment which shall govern the relationship of the parties as to
that country.

     2.  Appendix A to the License Agreement is hereby amended to add all
patents and patent applications related to the composition, manufacture or use
of modafinil that have been, or may be, filed or registered in the Latin
American Territory.

     3.  In consideration of the expansion of the Territory (and in addition to
any compensation otherwise payable with respect to countries outside of the
Latin American Territory under the terms of the License Agreement and the Supply
Agreement), Cephalon and Lafon will share equally any upfront fees or payments
that may be paid by third parties to Cephalon in consideration of being granted
distribution rights in any countries within the Latin American Territory.

     4.  Lafon will supply Cephalon with Compound under the terms of the Supply
Agreement in such quantities as may be necessary to meet all customer demand in

**Certain portions of this exhibit have been omitted based upon a request for
confidential treatment that has been filed with the Commission.  The omitted
portions have been filed separately with the Commission.
<PAGE>

Laboratoire L. Lafon
January 27, 2000
Page -2-

the Latin American Territory, and Cephalon will pay Lafon for such Compound an
amount equal to [**] percent [**] of Net Sales in the Latin American Territory.

     5.  Section 1.b. of Article V of the License Agreement is hereby amended
and restated in its entirety as follows:  Cephalon also will pay Lafon an amount
equal to [**] percent [**] of Net Sales in the Latin American Territory up to an
aggregate annual amount of US[**]; [**] percent [**] of Net Sales in the Latin
American Territory for that portion of the aggregate annual amount in excess of
US[**] but less than or equal to US[**]; and [**] percent [**] of Net Sales in
the Latin American Territory for that portion of the aggregate annual amount
that exceeds US[**].

     6.  Sections 3.b and 3.c of the License Agreement shall not apply to the
Latin American Territory.  Instead, the following provisions shall apply to
product registration activities in the Latin American Territory:

         3.b.  It is agreed that all product registrations (and applications)
within the Latin American Territory are to be in the name of Cephalon (or the
name of its Affiliate or sublicensee). Lafon shall take such actions as may be
required to identify Cephalon (or its Affiliate or sublicensee) as the applicant
and the holder of the product license in the respective country within the Latin
American Territory and, at the request of Cephalon, shall execute any documents
or instruments required under applicable law to confirm the authorization
granted hereunder and to apply for any other authorizations that may be required
to market the Licensed Product in the Latin American Territory, and/or join in
any such application by Cephalon, if required. Cephalon (or its Affiliate or
sublicensee) shall have the right to meet with the appropriate regulatory
authorities (including pricing and reimbursement authorities), but shall keep
Lafon informed of all such meetings and, upon request, shall provide Lafon with
copies of all relevant correspondence with such authorities.

         3.c.  Cephalon shall conduct, at its own expense, all necessary trials
for purposes of obtaining regulatory approvals of the Licensed Product in the
Latin American Territory.

         3.d.  Lafon will furnish Cephalon, upon request, with any copies of
correspondence or communications, whether occurring prior to the date hereof or
hereafter, that may exist between Lafon and any regulatory authorities in the
Latin American Territory related to application(s) for marketing approval for
the Licensed Product in the Latin American Territory.

     7.  Each of Cephalon and Lafon hereby restates its respective
representations and warranties made in the License Agreement and the Supply
Agreement, as each such agreement has been amended pursuant to this letter
agreement.  Lafon confirms that it is free to enter into this letter agreement,
without obligation to any third party.  Cephalon

**Certain portions of this exhibit have been omitted based upon a request for
confidential treatment that has been filed with the Commission.  The omitted
portions have been filed separately with the Commission.
<PAGE>

Laboratoire L. Lafon
January 27, 2000
Page -3-

shall not be responsible to any third party asserting a claim through Lafon with
respect to the development, manufacture or sale of Licensed Product for the
Latin American Territory.

     8.  Except as specifically supplemented by this letter agreement, all
provisions of each of the License Agreement and the Supply Agreement (in each
case, as amended prior to the date hereof) are confirmed to be and shall remain
in full force and effect.

     If the foregoing is acceptable, please indicate your agreement in the space
provided below.

                                        CEPHALON, INC.

                                        By: /s/ Frank Baldino, Jr.
                                           -----------------------------------
                                           Frank Baldino, Jr., Ph.D.
                                           President and Chief Executive Officer

AGREED, ACKNOWLEDGED AND ACCEPTED:

LABORATOIRE L. LAFON

By: /s/ F.C. Lafon
   -----------------------------
  F.C. Lafon
  Chief Executive Officer

**Certain portions of this exhibit have been omitted based upon a request for
confidential treatment that has been filed with the Commission.  The omitted
portions have been filed separately with the Commission.<PAGE>

                                                                 EXHIBIT 10.5(O)

                                              January  27, 2000

Laboratoire L. Lafon
19 Avenue du Professeur-Cadiot
94701 Maisons Alfort
France

        Re:  Amendment No. 8 to License Agreement and Supply Agreement
             ---------------------------------------------------------

Gentlemen:

     This letter agreement shall serve as an amendment to (a) the License
Agreement dated January 20, 1993, as previously amended (the "License
Agreement") between Cephalon, Inc. ("Cephalon") and Laboratoire L. Lafon
("Lafon"), and (b) the Supply Agreement dated January 20, 1993, as previously
amended (the "Supply Agreement") between Cephalon and Lafon.  All capitalized
terms not otherwise defined herein shall be used as defined in the License
Agreement.

     1.  The term "Territory," for all purposes under the License Agreement and
the Supply Agreement, is hereby expanded to include South Korea and Taiwan
(collectively, the "Asian Territory").

     2.  Appendix A to the License Agreement is hereby amended to add all
patents and patent applications related to the composition, manufacture or use
of modafinil that have been, or may be, filed or registered in the Asian
Territory.

     3.  In consideration of the expansion of the Territory (and in addition to
any compensation otherwise payable with respect to countries outside of the
Asian Territory, Territory under the terms of the License Agreement and the
Supply Agreement), Cephalon and Lafon will share equally any upfront fees or
payments that may be paid by third parties to Cephalon in consideration of being
granted distribution rights in any countries within the Asian Territory.

     4.  Lafon will supply Cephalon with Compound in such quantities as may be
necessary to meet all customer demand in the Asian Territory under the terms of
the Supply Agreement, and Cephalon will pay Lafon for such Compound an amount
equal to [**] percent [**] of Net Sales in the Asian Territory.

     5.  Section 1.b. of Article V of the License Agreement is hereby amended
and restated in its entirety as follows:  Cephalon also will pay Lafon an amount
equal to [**] percent [**] of Net Sales in the Asian Territory up to an
aggregate annual amount of US[**]; [**] percent [**] of Net Sales in the Asian
Territory for that portion of the

**Certain portions of this exhibit have been omitted based upon a request for
confidential treatment that has been filed with the Commission.  The omitted
portions have been filed separately with the Commission.
<PAGE>

Laboratoire L. Lafon
January 27, 2000
Page -2-

aggregate annual amount in excess of US[**] but less than or equal to US[**];
and [**] percent [**] of Net Sales in the Asian Territory for that portion of
the aggregate annual amount that exceeds US[**].

     6.  Sections 3.b and 3.c of the License Agreement shall not apply to the
Asian Territory.  Instead, the following provisions shall apply to product
registration activities in the Asian Territory:

         3.b.  It is agreed that all product registrations (and applications)
within the Asian Territory are to be in the name of Cephalon (or the name of its
Affiliate or sublicensee). Lafon shall take such actions as may be required to
identify Cephalon (or its Affiliate or sublicensee) as the applicant and the
holder of the product license in the respective country within the Asian
Territory and, at the request of Cephalon, shall execute any documents or
instruments required under applicable law to confirm the authorization granted
hereunder and to apply for any other authorizations that may be required to
market the Licensed Product in the Asian Territory, and/or join in any such
application by Cephalon, if required. Cephalon (or its Affiliate or sublicensee)
shall have the right to meet with the appropriate regulatory authorities
(including pricing and reimbursement authorities), but shall keep Lafon informed
of all such meetings and, upon request, shall provide Lafon with copies of all
relevant correspondence with such authorities.

         3.c.  Cephalon shall conduct, at its own expense, all necessary trials
for purposes of obtaining regulatory approvals of the Licensed Product in the
Asian Territory.

         3.d.  Lafon will furnish Cephalon, upon request, with any copies of
correspondence or communications, whether occurring prior to the date hereof or
hereafter, that may exist between Lafon and any regulatory authorities in the
Asian Territory related to application(s) for marketing approval for the
Licensed Product in the Asian Territory.

     7.  Each of Cephalon and Lafon hereby restates its respective
representations and warranties made in the License Agreement and the Supply
Agreement, as each such agreement has been amended pursuant to this letter
agreement.  Lafon confirms that it is free to enter into this letter agreement,
without obligation to any third party.  Cephalon shall not be responsible to any
third party asserting a claim through Lafon with respect to the development,
manufacture or sale of Licensed Product for the Asian Territory.

     8.  Except as specifically supplemented by this letter agreement, all
provisions of each of the License Agreement and the Supply Agreement (in each
case, as amended prior to the date hereof) are confirmed to be and shall remain
in full force and effect.

**Certain portions of this exhibit have been omitted based upon a request for
confidential treatment that has been filed with the Commission.  The omitted
portions have been filed separately with the Commission.
<PAGE>

Laboratoire L. Lafon
January 27, 2000
Page -3-

     If the foregoing is acceptable, please indicate your agreement in the space
provided below.

                                        CEPHALON, INC.

                                        By: /s/ Frank Baldino, Jr.
                                           -----------------------------------
                                           Frank Baldino, Jr., Ph.D.
                                           President and Chief Executive Officer

AGREED, ACKNOWLEDGED AND ACCEPTED:

LABORATOIRE L. LAFON

By: /s/ F.C. Lafon
   -----------------------------
   F.C. Lafon
   Chief Executive Officer

**Certain portions of this exhibit have been omitted based upon a request for
confidential treatment that has been filed with the Commission.  The omitted
portions have been filed separately with the Commission.

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