<PAGE>

                                                                  EXHIBIT 4.3(G)

THE CONFIDENTIAL PORTIONS OF THIS EXHIBIT HAVE BEEN FILED SEPARATELY WITH THE
SECURITIES AND EXCHANGE COMMISSION PURSUANT TO A CONFIDENTIAL TREATMENT REQUEST
IN ACCORDANCE WITH RULE 24B-2 OF THE SECURITIES AND EXCHANGE ACT OF 1934, AS
AMENDED. REDACTED PORTIONS OF THIS EXHIBIT ARE MARKED BY AN ***.

              FIRST AMENDMENT TO THE CO-PROMOTION AGREEMENT BETWEEN
                          VERNALIS DEVELOPMENT LIMITED
                                       AND
                            ENDO PHARMACEUTICALS INC.
                               DATED JULY 1, 2005

This First Amendment to the Co-Promotion Agreement by and between Vernalis
Development Limited ("Vernalis") and Endo Pharmaceuticals Inc. ("Endo") (the
"First Amendment") is effective as of December 12, 2005.

WHEREAS, Vernalis and Endo entered into a Co-Promotion Agreement dated July 1,
2005 (the "Co-Promotion Agreement"), governing the terms under which Endo and
Vernalis would work together to promote the Product in the USA; and

WHEREAS, Vernalis and Endo wish to amend the Co-Promotion Agreement as more
fully set forth below;

NOW THEREFORE, for good and valuable consideration, the receipt of which is
hereby acknowledged, the parties hereto agree to amend the Co-Promotion
Agreement as follows:

1.   Unless set forth herein, the capitalized terms contained in this First
     Amendment shall have the meanings set forth in the Co-Promotion Agreement.

2.   The following Definition is hereby added to Section 1:

     "Tertiary Detail" means a Product Detail presented with lesser prominence
     than a Secondary Detail but more prominence than mere inclusion in a
     product list.

3.   The following is inserted between "not" and "(x)" in Section 2.1(b):

     "as determined by reference to the process set out in Section 2.1(c)".

4.   The following is added to the Co-Promotion Agreement as Section 2.1(c):

     "(c) Process for determining Non-Compete Obligations:

          To assist the parties in determining which products fall under the
          non-compete provisions set forth in Section 2.1(b), it is agreed that:

          (i)  Endo shall provide to Vernalis an initial and updated summaries
               of all Endo's products either marketed in the USA or under
               development that have successfully completed clinical proof of
               principle as at the date of each

<PAGE>

               summary. Such summaries shall include at least: (a) the name of
               the product (if marketed) and the class of the molecule and (b)
               the indication or indications for which the product is being
               marketed or developed (the "Non-compete Summary").

               Endo shall provide a Non-compete Summary to Vernalis at the
               following times:

               (A)  within *** business days of the date of this First
                    Amendment.

               (B)  no later than *** business days following March 1 and
                    September 1 of each calendar year during the term of the
                    Co-Promotion Agreement.

               (C)  additionally at any time within *** business days of any
                    request by Vernalis but not more than once during any period
                    March 1 - August 31 or September 1 - February 29 in any
                    year.

          (ii) Each updated Non-compete Summary shall supercede all previous
               Non-compete Summaries other than with respect to the provisions
               of Section 2.1 (c) (vi) (B).

          (iii) In the event Endo fails to provide to Vernalis a Non-compete
               Summary by any due date provided in Section 2.1 (c) (i) (B) or
               (C), Endo may not provide a Non-compete Summary to Vernalis
               (unless subsequently requested by Vernalis under Section 2.1 (c)
               (i) (C)) until the next scheduled due date under Section 2.1 (c)
               (i) (B) and the last issued Non-Compete Summary shall continue to
               govern until such time.

          (iv) On any date that Endo publicly announces an acquisition (via
               license, purchase or otherwise) of a product which has
               successfully completed clinical proof of principle for any
               indication(s), such product and indication(s) shall automatically
               be deemed to be included on the most recent Non-compete Summary
               as of the date of such public announcement.

          (v)  Save as provided in Section 2.1 (c) (vi) Vernalis may not use the
               Specialty Sales Personnel to Commercialise in the USA any product
               for any indication which is in direct competition with an Endo
               product and its applicable indication for so long as such product
               and indication is included on a Non-compete Summary provided by
               Endo under Section 2.1 (c) (i) (A), (B) or (C), or deemed to be
               included on a Non-compete Summary pursuant to Section 2.1 (c)
               (iv).

          (vi) Notwithstanding anything herein to the contrary, Vernalis shall
               not be prohibited at any time from using the Specialty Sales
               Personnel to Commercialise in the USA any product for which
               rights to Commercialise were acquired by Vernalis:

               (A)  prior to receipt from Endo of any Non-compete Summary which
                    listed an Endo product and applicable indication with which
                    Vernalis' product is in direct competition.

               (B)  within six (6) months following Vernalis' receipt of a
                    Non-compete Summary under Section 2.1 (c) (i) (C) provided
                    that the Non-compete Summary did not list an Endo product
                    and applicable indication with which Vernalis' product is in
                    direct competition. This shall apply regardless of any
                    updated Non-compete Summary provided to Vernalis


                                       2

<PAGE>

                    by Endo under Section 2.1 (c) (i) (B) before expiry of this
                    six (6) month period.

                    By way of example, if Endo provides to Vernalis a
                    Non-compete Summary under Section 2.1 (c) (i) (C) on August
                    15, 2006 which does not include a product and applicable
                    indication which is in direct competition with Product X,
                    Vernalis may use the Specialty Sales Personnel to
                    Commercialise Product X in the USA if Vernalis enters into
                    an agreement with a third party for such rights to Product X
                    before February 16, 2007 and this shall apply even if an
                    Endo product in direct competition with Product X is listed
                    on the updated Non-compete Summary provided by Endo under
                    Section 2.1 (c) (i) (B) on September 1, 2006.

          (vii) At no time is Vernalis prohibited from using sales
               representatives other than the Specialty Sales Personnel to
               Commercialise any product in the USA whether or not such product
               is in direct competition with an Endo product for an indication
               listed on a Non-compete Summary then in effect.

5.   Section 3.1(a) is amended to add the following phrase to the end of the
     last sentence in such section:

     "; provided that (i) at least *** Product Details made by Vernalis shall be
     Primary Details and (ii) no more than *** Product Details made by Vernalis
     shall be Tertiary Details."

6.   The following sentences shall be added after the second sentence in Section
     3.3(b)(iii):

     "In the event that Vernalis fails to provide at least *** Primary Details
     and/or provides more than *** Tertiary Details as set forth in Section
     3.1(a) in any two consecutive quarters, Endo shall, as applicable, (i)
     reduce the reimbursement payment to Vernalis in such second quarter by the
     number of Primary Details that fell short of the *** requirement in the
     second quarter multiplied by the Cost Per Detail and/or (ii) not reimburse
     Vernalis the Cost Per Detail for any Tertiary Detail that exceeded the ***
     requirement in the second quarter. Until Vernalis remedies the deficiencies
     set forth in this Section 3.3(b)(iii), Endo may continue to reduce
     reimbursement payments in accordance with the provisions of this Section
     3.3(b)(iii) for each consecutive quarter that the deficiencies continue."

7.   The table in Section 3.3(b)(iii) is hereby deleted and replaced with the
     following to reflect the amendment to the License as of the date hereof:

<TABLE>
<CAPTION>
YEARS                    2006   2007   2008   2009   2010
-----                    ----   ----   ----   ----   ----
<S>                      <C>    <C>    <C>    <C>    <C>
Maximum no. of Details   ***    ***    ***    ***    ***
to be reimbursed

% of Cost per Detail     ***    ***    ***    ***    ***
where all Vernalis
Specialty Sales
Personnel Detail only
the Product

% of Cost per Detail     ***    ***    ***    ***    ***
where any Vernalis
Specialty Sales
Personnel Detail one
or more other products
</TABLE>


                                       3

<PAGE>

8.   Exhibit A to the Co-Promotion Agreement is hereby deleted in its entirety
     and replaced with the attached Exhibit A.

9.   Save as herein provided, all other terms and conditions of the Co-Promotion
     Agreement remain in full force and effect as originally written therein.

     IN WITNESS WHEREOF, the parties have duly executed this First Amendment as
of the first date written above.

                                        ENDO PHARMACEUTICALS INC.


                                        By: /s/ PETER LANKAU
                                            ------------------------------------
                                        Name: Peter Lankau
                                        Title: President and CEO


                                        VERNALIS DEVELOPMENT LTD.


                                        By: /s/ JOHN A. D. SLATER
                                            ------------------------------------
                                        Name: John A D Slater
                                        Title: Director


                                       4

<PAGE>

                                    Exhibit A

                        Example of Vernalis Detail Report

                  DETAIL REPORT FOR QUARTER ENDED ____________

                               THIS IS AN INVOICE

<TABLE>
<CAPTION>
                                          ACTUAL #   BUDGET #
                                          --------   --------
<S>                                       <C>        <C>
Total number of Product Details

Total number of Neurology Details

Total number of Neurology Details to
   Target Healthcare Professionals:
   Primary Details
   Secondary Details
   Tertiary Details

Total number of Primary Care Physician
   Details

Total number of Primary Care Physician
   Details to Target Healthcare
   Professionals:
   Primary Details
   Secondary Details
   Tertiary Details
Total number of OBGYN Details

Total number of OBGYN Details to Target
   Healthcare Professionals:
   Primary Details
   Secondary Details
   Tertiary Details
</TABLE>