AGREEMENT

                                                                      EXHIBIT 13

                                   AGREEMENT

     AGREEMENT, dated as of July 17, 2000 (this "Agreement"), by and between
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ENDO PHARMACEUTICALS HOLDINGS INC., a Delaware corporation ("Holdings"), and
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ENDO PHARMA LLC, a Delaware limited liability company ("Endo LLC").
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                                  WITNESSETH:

     WHEREAS, Holdings, Endo Inc., a Delaware corporation and a newly formed,
wholly owned subsidiary of Holdings ("Sub"), and Algos Pharmaceutical
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Corporation, a Delaware corporation (the "Algos"), entered into an agreement and
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plan of merger, dated as of November 26, 1999 (as amended and restated as of
March 3, 2000 and as amended on April 17, 2000, the "Merger Agreement"), whereby
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Algos will merge with and into Sub;

     WHEREAS, Endo LLC was formed in order to complete certain of the
transactions contemplated by the Merger Agreement;

     WHEREAS, the Board of Directors of Holdings and the Board of Managers of
Endo LLC have each determined that this Agreement is advisable and in the best
interest of their stockholders and unitholders, respectively, and is in
furtherance of and consistent with their respective long-term business
strategies;

     WHEREAS, the Board of Directors of Holdings and the Board of Managers of
Endo LLC have each approved this Agreement upon the terms and subject to the
conditions set forth herein;

     NOW, THEREFORE, in consideration of the premises, representations,
warranties and agreements herein contained, the parties hereto agree as follows:

          FIRST, Endo LLC hereby agrees to comply with Section 5.21 of the
     Merger Agreement, insofar as such section is applicable to Endo LLC,
     thereby having the effect of making Endo LLC a party to the Merger
     Agreement for purposes of such section; and


          SECOND, Endo LLC hereby further agrees that, with respect to its
     agreement in clause FIRST above, it will be subject to Sections 8.6, 8.7
     and 8.9 of the Merger Agreement.

     This Agreement may be executed in counterparts, all of which shall be
considered one and the same agreement, and shall become effective when one or
more counterparts have been signed by each of the parties hereto and delivered
to the other party.


                             ENDO PHARMACEUTICALS HOLDINGS INC.

                             By: /s/ Xxxxx X. Xxxxx
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                                Name:  Xxxxx X. Xxxxx
                                Title: President & Chief Executive Officer


                             ENDO PHARMA LLC

                             By: /s/ Xxxxxxx X. Xxxxx
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                                Name:  Xxxxxxx X. Xxxxx
                                Title: Chief Financial Officer

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