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                                                                   Exhibit 10.8

                [PRAECIS PHARMACEUTICALS INCORPORATED LETTERHEAD]




                                                              January 30, 2001




VIA FACSIMILE AND FEDEX
-----------------------

Sanofi-Synthelabo Inc.
90 Park Avenue
New York, NY 10016
Facsimile No. 212-551-4921
Attention: John M. Spinnato,
           Senior Vice President and General Counsel

                  Re:   Amendment to Warrant for the Purchase of Shares of
                        Common Stock of PRAECIS PHARMACEUTICALS
                        INCORPORATED
                        ------------

Dear Sirs:

                  This letter will constitute an amendment, effective as of the
date hereof, to the Warrant for the Purchase of Shares of Common Stock (the
"Warrant") originally issued by PRAECIS PHARMACEUTICALS INCORPORATED (the
"Company") to Sylamerica, Inc., now known as Sanofi-Synthelabo Inc.
("Sanofi-Synthelabo"), pursuant to the Stock and Warrant Purchase Agreement,
dated as of May 13, 1997, between Sylamerica, Inc. and the Company. Capitalized
terms used herein and not otherwise defined shall have the meanings ascribed to
them in the Warrant.

         The parties hereby agree that the Warrant is amended as follows:

         -  Section 1. of the existing Warrant is renumbered as "Section 1.(a)".

         -  A new paragraph (b) to Section 1. is added to the Warrant and reads
            in its entirety as follows:

            1.(b)   Notwithstanding paragraph (a) of this Section 1., in lieu of
            payment of the Exercise Price, the Holder may exercise this Warrant,
            in whole or in part, by surrender of this Warrant, together with a
            Cashless Exercise Form in the form attached hereto as Exhibit II,
            duly executed


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Sanofi-Synthelabo Inc.
Page 2


            (a "Cashless Exercise"). Acceptance by the Company of such
            presentation and surrender shall be deemed a waiver of the Holder's
            obligation to pay all or any part of the Exercise Price for the
            Shares covered by such exercise. In the event of a Cashless
            Exercise, the Holder shall exchange this Warrant for that number of
            shares of Common Stock (rounded up to the nearest whole share,
            notwithstanding the provisions of Section 2 of this Warrant relating
            to the treatment of fractional shares) determined by multiplying the
            number of Shares for which the Warrant is being so exercised by a
            fraction, the numerator of which shall be the positive difference
            (if any) between the then current market price per share of the
            Common Stock and the Exercise Price, and the denominator of which
            shall be the then current market price per share of Common Stock.
            For purposes of any computation under this Section 1(b), the then
            current market price per share of Common Stock shall be deemed to be
            the closing sales price per share of the Common Stock on the
            principal public trading market on which the Common Stock is then
            traded on the business day immediately preceding the date of the
            Cashless Exercise, or if the Common Stock is not then traded on any
            public trading market, the fair market price of the Common Stock as
            at such date, as determined in good faith by the Company's Board of
            Directors.

            - A new Exhibit II is added to the Warrant and reads in its
              entirety as provided in the attached form of Exhibit II.


            All other terms and provisions of the Warrant will remain in full
force and effect.

            The Company and Sanofi-Synthelabo acknowledge, by their respective
signatures below, that no commission, remuneration or other consideration was
paid or given by them in connection with the matters set forth herein.



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Sanofi-Synthelabo Inc.
Page 3

            Please acknowledge your acceptance of and agreement with the
foregoing by signing below in the space provided and returning a copy of this
letter to Kevin F. McLaughlin at the address set forth above.

                                                Very truly yours,

                                                PRAECIS PHARMACEUTICALS
                                                INCORPORATED


                                                By /s/ Kevin F. McLaughlin
                                                   ----------------------------
                                                   Name: Kevin F. McLaughlin
                                                   Title:  Sr. V.P. and CFO

Enclosure

cc:      Coudert Brothers
         1114 Avenue of the Americas
         New York, NY  10036-7703
         Attention: James Colihan, Esq.

Accepted and Agreed:

SANOFI-SYNTHELABO INC.


By /s/ Gregory Irace
   -----------------------------------------
    Name:  Gregory Irace
    Title: Vice President and CFO


By /s/ Richard H. Thomson
   -----------------------------------------
    Name:  Richard H. Thomson
    Title: Vice President and Treasurer

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                               FORM OF EXHIBIT II


                                                                     Exhibit II
                                                                     ----------

                          CASHLESS ELECTION TO EXERCISE

    The undersigned hereby irrevocably exercises its right to subscribe for and
purchase from PRAECIS PHARMACEUTICALS INCORPORATED, a Delaware corporation,
__________ fully paid, validly issued and nonassessable shares of Common Stock
covered by the within Warrant. The undersigned hereby elects to pay the Exercise
Price pursuant to the Cashless Exercise provisions of the Warrant, as provided
for in Section 1(b) of such Warrant, as amended to date.


                                                    SANOFI-SYNTHELABO INC.

                                                    By________________________
                                                        Name:
                                                        Title:

Date:________________

*********************************************************

INSTRUCTIONS FOR ISSUANCE OF STOCK CERTIFICATE

(if other than to the registered holder of the within Warrant)

Name:____________________________

Address:__________________________
        __________________________
        __________________________
        __________________________


Social Security or
Taxpayer Identification Number:
                               ---------------------------