<PAGE>
                                                                   Exhibit 10.7

                              WAIVER AND AMENDMENT

         This Waiver and Amendment, dated as of January 26, 2001 (this "Waiver
and Amendment"), waives and amends certain provisions of the Stock and Warrant
Purchase Agreement, dated as of May 13, 1997 (the "Stock and Warrant Purchase
Agreement"), between SYLAMERICA, INC. (now Sanofi-Synthelabo Inc.)
("Sanofi-Synthelabo") and PRAECIS PHARMACEUTICALS INCORPORATED (the "Company").
Capitalized terms used herein and not otherwise defined shall have the meanings
ascribed to them in the Stock and Warrant Purchase Agreement. All references
herein to sections shall be understood to be references to sections of the Stock
and Warrant Purchase Agreement.

         WHEREAS, on January 18, 2001, the Company, in accordance with Section
6.1 of the Stock and Warrant Purchase Agreement, provided notice to
Sanofi-Synthelabo of a proposed firm commitment underwritten public offering of
shares of its common stock for which a Registration Statement on Form S-1 was
filed with the Securities and Exchange Commission on January 26, 2001 (the
"Offering");

         WHEREAS, the Company has requested that Sanofi-Synthelabo waive its
right to include Registrable Securities in the Offering pursuant to Section 6.1
of the Stock and Warrant Purchase Agreement, in consideration for the Company
agreeing to amend the Stock and Warrant Purchase Agreement as provided herein;



<PAGE>

         WHEREAS, Sanofi-Synthelabo has agreed to waive its right to include
Registrable Securities in the Offering; and

         WHEREAS, in consideration of Sanofi-Synthelabo's waiver of such right,
the Company has agreed to amend Section 6.1(c) of the Stock and Warrant Purchase
Agreement as provided herein.


         NOW, THEREFORE, the parties hereto hereby agree as follows:

         1. WAIVER. Sanofi-Synthelabo hereby irrevocably waives any right it may
otherwise have pursuant to Section 6.1 of the Stock and Warrant Purchase
Agreement to include Registrable Securities in the Offering.

         2. AMENDMENT. Section 6.2(c) of the Stock and Warrant Purchase
Agreement is hereby amended in its entirety to read as follows:


         (c) If a registration undertaken by PRAECIS involves an Underwritten
         Offering, Purchaser, if and only if any Registrable Securities are
         included in such registration, will, if requested by the Managing
         Underwriter, enter into an agreement not to effect any public sale or
         distribution, including any sale pursuant to Rule 144 under the
         Securities Act, of any Registrable Securities, or of any security
         convertible into or exchangeable or exercisable for any Registrable
         Securities (other than as part of such Underwritten Offering), without
         the consent of the Managing Underwriter, during a period commencing on
         the effective date of such registration and ending a number of calendar
         days thereafter not exceeding 180 as the Board of Directors of PRAECIS
         and the Managing Underwriter shall reasonably determine is required to
         effect a successful offering.


         3. APPLICABILITY OF AMENDMENT TO THE OFFERING. For the avoidance of
doubt, the parties acknowledge that the amendment of Section 6.2(c) of the Stock



                                       2

<PAGE>

and Warrant Purchase Agreement effected by Section 2 hereof shall apply with
respect to the Offering and, accordingly, Purchaser shall have no obligation to
enter into an agreement referred to in Section 6.2(c), as amended, in connection
with the Offering.

         4. PARTIES IN INTEREST. This Waiver and Amendment shall be binding upon
and inure solely to the benefit of each party hereto and its respective
successors and assigns. Nothing in this Waiver and Amendment, express or
implied, is intended to or shall confer upon any other person any rights,
benefits or remedies of any nature whatsoever under or by reason of this Waiver
and Amendment.

         5. COUNTERPARTS. This Waiver and Amendment may be executed in
counterparts, each of which shall be deemed an original but which together shall
constitute one and the same instrument.

         6. GOVERNING LAW. This Waiver and Amendment shall be governed by and
construed in accordance with the laws of the State of Delaware, without giving
effect to the conflicts of law principles thereof.


            [The remainder of this page is intentionally left blank.]


                                       3
<PAGE>


         IN WITNESS WHEREOF, each of the parties has executed this Waiver and
Amendment as of the date first above written.

                               PRAECIS PHARMACEUTICALS INCORPORATED


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


                               SANOFI-SYNTHELABO INC.


                               By /s/ Gregory Irace
                                  -----------------------------
                               Name: Gregory Irace
                               Title: Vice President and Chief Financial Officer


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





                                      4