EXHIBIT 4.1

                          AMENDMENT TO RIGHTS AGREEMENT

      THIS AMENDMENT (this "AMENDMENT"), is entered into as of August 24, 2001,
by and between TRIANGLE PHARMACEUTICALS, INC., a Delaware corporation (the
"COMPANY"), and AMERICAN STOCK TRANSFER & TRUST COMPANY, as Rights Agent (the
"RIGHTS AGENT").

                                    RECITALS

      A. The Company and the Rights Agent are parties to a Rights Agreement
dated as of February 1, 1999, as amended by the Amendment to Rights Agreement
dated as of June 2, 1999 (as so amended, the "RIGHTS AGREEMENT").

      B. Warburg Pincus Private Equity VIII, L.P. ("WARBURG") and the Company
have entered into a Purchase Agreement dated as of August 24, 2001 (the "WARBURG
STOCK PURCHASE AGREEMENT"), pursuant to which Warburg is to purchase shares of
Company Common Stock.

      C. The Board of Directors of the Company has determined that an amendment
to the Rights Agreement as set forth herein is necessary and desirable to
reflect the foregoing and certain other matters and the Company and the Rights
Agent desire to evidence such amendment in writing.

            Accordingly, the parties agree that:

      1. AMENDMENT TO DEFINITION OF "ACQUIRING PERSON" SET FORTH IN SECTION
1(A). The definition of "Acquiring Person" set forth in Section 1(a) of the
Rights Agreement is amended to read in its entirety as follows:

      (a) "ACQUIRING PERSON" shall mean any Person (as such term is hereinafter
      defined) who or which, together with all Affiliates and Associates (as
      such terms are hereinafter defined) of such Person, shall be the
      Beneficial Owner (as such term is hereinafter defined) of 15% or more of
      the shares of Common Stock of the Company then outstanding but shall not
      include (1) the Company, any Subsidiary (as such term is hereinafter
      defined) of the Company, any employee benefit plan of the Company or any
      Subsidiary of the Company, or any entity holding shares of Common Stock
      for or pursuant to the terms of any such plan, (2) during the Interim
      Period (as defined below), Abbott Laboratories, an Illinois corporation
      (the "PERMITTED INVESTOR"), or any U.S. wholly-owned subsidiaries of the
      Permitted Investor (collectively with the Permitted Investor, the
      "INVESTOR GROUP") but only to the extent that the Permitted Investor may
      be deemed a Beneficial Owner of the Shares (as defined below) as a result
      of the Permitted Investor entering into the Abbott Stock Purchase
      Agreement (as defined below), unless the Permitted Investor becomes the
      Beneficial Owner (as defined in the Abbott Rights Agreement (as defined
      below)) of any additional securities of the Company other than the Shares
      during the Interim Period, (3) during the Threshold Period (as hereinafter
      defined), the Investor Group, or (4) Warburg Pincus Private Equity VIII,
      L.P., together with all its Affiliates ("WARBURG"), unless and until
      Warburg becomes the Beneficial Owner of more than the Permitted Percentage
      (as hereinafter defined); provided, however, that if Warburg files a
      Schedule 13D (or comparable or successor form or report) under the
      Exchange Act disclosing that Warburg holds the Common Stock for any
      purpose of, or with the effect of, causing the Company to enter into a
      merger, consolidation, business combination, acquisition, restructuring,
      recapitalization, tender or

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      exchange offer, or similar transaction involving the Company, or its
      securities or a material portion of its assets (other than such a
      transaction approved by the Board of Directors of the Company), or in
      connection with or as a participant in any transaction under Rule 13d-3(b)
      under the Exchange Act and Warburg's Beneficial Ownership of Common Stock
      equals fifteen percent (15%) or more, then Warburg shall become an
      Acquiring Person. For purposes of this Agreement, the "INTERIM PERIOD"
      shall mean such period commencing as of the date of the Abbott Stock
      Purchase Agreement and ending on the earlier of (x) the "EFFECTIVE DATE"
      as such term is defined in the Collaboration Agreement (as defined below)
      and (y) termination of the Abbott Stock Purchase Agreement. For purposes
      of this Agreement, the "THRESHOLD PERIOD" shall mean such period
      commencing as of the Effective Date under the Collaboration Agreement and
      ending on the earlier of (xx) such time as the Permitted Investor holds
      less than the Minimum Purchaser Interest in the Company (as defined in
      Section 1.11 of the Abbott Rights Agreement), (yy) such time as the
      Investor Group's Beneficial Ownership (as the term Beneficial Ownership is
      defined in the Abbott Rights Agreement) exceeds the Beneficial Ownership
      Limitation (as defined in Section 5.2(a) of the Abbott Rights Agreement),
      and (zz) such time as the Permitted Investor's rights under Section 7.1 of
      the Abbott Rights Agreement have terminated pursuant to the terms of the
      Abbott Rights Agreement. Notwithstanding the foregoing:

                (i) no Person shall become an "Acquiring Person" as the result
      of an acquisition of shares of Common Stock by the Company which, by
      reducing the number of shares outstanding, (a) increases the proportionate
      number of shares beneficially owned by such Person to 15% or more (or in
      the case of Warburg, more than the Permitted Percentage) of the shares of
      Common Stock of the Company then outstanding, or, (b) in the case of the
      Investor Group during the Interim Period, increases the proportionate
      number of shares which may be deemed beneficially owned by the Investor
      Group, unless the Permitted Investor becomes the Beneficial Owner (as
      defined in the Abbott Rights Agreement) of any additional securities of
      the Company other than the Shares during the Interim Period, or, (c) in
      the case of the Investor Group during the Threshold Period, increases the
      proportionate number of shares Beneficially Owned (as defined in the
      Abbott Rights Agreement) by the Investor Group to more than the Beneficial
      Ownership Limitation; provided, however, that if by reason of share
      purchases by the Company, (1) a Person shall become the Beneficial Owner
      of 15% or more (or in the case of Warburg, more than the Permitted
      Percentage) of the shares of Common Stock of the Company then outstanding
      or, (2) in the case of the Investor Group, the Investor Group's ownership
      increases the Investor Group's Beneficial Ownership (as defined in the
      Abbott Rights Agreement) by any amount during the Interim Period or
      results in the Investor Group's Beneficial Ownership (as defined in the
      Abbott Rights Agreement) to exceed the Beneficial Ownership Limitation at
      any time during the Threshold Period, and, in the case of (1) or (2)
      above, such Person or Investor Group shall, after such share purchases by
      the Company, become the Beneficial Owner of any additional shares of
      Common Stock of the Company, then such Person, including the Investor
      Group, shall be deemed to be an "Acquiring Person" hereunder; and

                (ii) if the Board of Directors of the Company determines in good
      faith that a Person who would otherwise be an "Acquiring Person" as
      defined pursuant to the foregoing provisions of this paragraph (a), has
      become such inadvertently, and such Person divests as promptly as
      practicable a sufficient number of shares of Common Stock so that such
      Person would no longer be an "Acquiring Person" (as defined


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      pursuant to the foregoing provisions of this paragraph (a)), then such
      Person shall not be deemed to be an "Acquiring Person" for any purpose of
      this Agreement."

      2. AMENDMENT TO DEFINITION OF "TRIGGERING EVENT" SET FORTH IN SECTION
1(oo). The definition of "Triggering Event" set forth in Section 1 (oo) of the
Rights Agreement is amended to add the following sentence to the end thereof:

      "Notwithstanding anything in this Rights Agreement to the contrary, a
      Triggering Event shall not be deemed to have occurred by virtue of (i) the
      Warburg Stock Purchase Agreement or by virtue of any of the transactions
      contemplated thereby, or (ii) the Abbott Stock Purchase Agreement and the
      Related Agreements (as such term is defined in the Abbott Stock Purchase
      Agreement) or by virtue of any of the transactions contemplated thereby
      (excluding during the Interim Period, any purchases which result in the
      Permitted Investor becoming the Beneficial Owner (as defined in the Abbott
      Rights Agreement) of any securities of the Company in addition to the
      Shares and excluding, during the Threshold Period, any purchases permitted
      by Section 5.3 of the Abbott Rights Agreement or otherwise which cause the
      Permitted Investor's Beneficial Ownership (as defined in Section 5.4 of
      the Abbott Rights Agreement) of shares of Common Stock to exceed 21% of
      the then total outstanding shares of Common Stock of the Company)."

      3. AMENDMENT TO SECTION 1. The following definitions are hereby added to
the end of Section 1 as Sections 1(tt) and 1(uu):

      "(tt) "WARBURG STOCK PURCHASE AGREEMENT" shall mean that certain Purchase
      Agreement dated as of August 24, 2001, by and between the Company and
      Warburg Pincus Private Equity VIII, L.P.

      (uu) "PERMITTED PERCENTAGE" shall mean the lesser of (a) forty percent
      (40%) of the capital stock of the Company then outstanding having the
      right to vote or (b) (i) for the period effective immediately prior to the
      Initial Closing (as such term is defined in the Warburg Stock Purchase
      Agreement) and continuing until immediately prior to the Second Closing
      (as such term is defined in the Warburg Stock Purchase Agreement), or if
      no Second Closing occurs, then indefinitely, the percentage determined by
      dividing 9,628,002 by the number of shares of Common Stock of the Company
      outstanding (giving effect to the shares issued or to be issued in the
      Initial Closing) and (ii) for the period effective immediately prior to
      the Second Closing and continuing indefinitely, the sum of (x) the
      percentage determined by dividing the number of shares of Common Stock of
      which Warburg is then the Beneficial Owner, after giving effect to the
      purchases of shares of Common Stock of the Company by Warburg at such
      Second Closing, by the number of shares of Common Stock of the Company
      outstanding (giving effect to the shares issued or to be issued in the
      Second Closing), plus (y) five percent (5%); provided, however, that the
      Permitted Percentage shall be reestablished on such date (the "Initial
      Reset Date") as, through sales or other transfers (other than transfers to
      Affiliates of Warburg) by Warburg, Warburg becomes the Beneficial Owner of
      less than seventy-five percent (75%) of the number of shares of Common
      Stock of the Company purchased by Warburg pursuant to the Warburg Stock
      Purchase Agreement, and thereafter reestablished on each date (each such
      date along with the Initial Reset Date being referred to herein as a
      "Reset Date") on which, through sales or other transfers (other than
      transfers to Affiliates of Warburg) by Warburg, the number of shares of
      Common Stock of the Company Beneficially Owned by Warburg decreases from
      any


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      previously established Permitted Percentage by more than two percent (2%)
      of the then outstanding shares of Common Stock of the Company, to that
      percentage of Common Stock of the Company Beneficially Owned by Warburg on
      such Reset Date, plus five percent (5%). For purposes of clause (b) of
      this Section 1(uu), determinations at any given point in time of the
      Permitted Percentage or of the ownership of shares of Common Stock of the
      Company by Warburg in relation to the then applicable Permitted Percentage
      shall be calculated in accordance with Section 1(d)(iv) hereof.
      Notwithstanding the foregoing, the Permitted Percentage shall be adjusted
      at any time so as to permit Warburg to exercise its subscription rights
      under Section 8.5 of the Warburg Stock Purchase Agreement."

      4. AMENDMENT OF SECTION 3(a). The first sentence of Section 3(a) of the
Rights Agreement is amended to read in its entirety as follows:

      "Until the earlier of (i) the Close of Business on the Shares Acquisition
      Date and (ii) the Close of Business on the tenth Business Day (or such
      later date as may be determined by action of the Company's Board of
      Directors prior to such time as any Person becomes an Acquiring Person and
      of which the Company will give the Rights Agent prompt written notice)
      after the date that a tender or exchange offer by any Person (other than
      the Company, any Subsidiary of the Company, any employee benefit plan of
      the Company or of any Subsidiary of the Company or any entity holding
      shares of Common Stock for or pursuant to the terms of any such plan) is
      first published or sent or given within the meaning of Rule 14d-4(a) of
      the Exchange Act Regulations or any successor rule or of the first public
      announcement of the intention of any Person (other than the Company, any
      Subsidiary of the Company, any employee benefit plan of the Company or of
      any Subsidiary of the Company or any entity holding shares of Common Stock
      for or pursuant to the terms of any such plan) to commence, a tender or
      exchange offer, if upon consummation thereof such Person would be the
      Beneficial Owner of 15% or more (or in the case of Warburg, more than the
      PERMITTED PERCENTAGE) of the shares of Company Common Stock then
      outstanding, or, in the case of the Investor Group during the Interim
      Period, the Investor Group purchases any securities of the Company which
      result in the Permitted Investor becoming the Beneficial Owner (as defined
      in the Abbott Rights Agreement) of any securities of the Company in
      addition to the Shares, or in the case of the Investor Group during the
      Threshold Period, the Permitted Investor's Beneficial Ownership (as
      defined in the Abbott Rights Agreement) would exceed the Beneficial
      Ownership Limitation (as defined in the Abbott Rights Agreement), (the
      earlier of (i) and (ii) above being the "DISTRIBUTION DATE"), (x) the
      Rights will be evidenced (subject to the provisions of Section 3(b)
      hereof) by the certificates for shares of Common Stock registered in the
      names of the holders thereof (which certificates shall also be deemed to
      be Rights Certificates) and not by separate Rights Certificates, and (y)
      the right to receive Rights Certificates will be transferable only in
      connection with the transfer of shares of Common Stock."

      5. MISCELLANEOUS. This Amendment shall be deemed to be a contract made
under the laws of the State of Delaware and for all purposes shall be governed
by and construed in accordance with the laws of such State applicable to
contracts to be made and performed entirely within such State. This Amendment
may be executed in any number of counterparts, each of such counterparts shall
for all purposes be deemed to be an original, and all such counterparts shall
together constitute but one and the same instrument. If any term, provision,
covenant or restriction of this Amendment is held by a court of competent
jurisdiction or other authority to be invalid, void or unenforceable, the
remainder of the terms, provisions, covenants


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and restrictions of this Amendment shall remain in full force and effect and
shall in no way be affected, impaired or invalidated.

                          (SIGNATURE ON FOLLOWING PAGE)


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              (SIGNATURE PAGE TO THE AMENDMENT TO RIGHTS AGREEMENT)

      IN WITNESS WHEREOF, the parties hereto have caused this Amendment to
Rights Agreement to be duly executed and attested, all as of the day and year
first above written.

ATTEST:                                 TRIANGLE PHARMACEUTICALS, INC.

By:         /s/ Andrew Finkle           By:         /s/ Chris A. Rallis
     --------------------------------        -----------------------------------
     Name:  Andrew Finkle                    Name:  Chris A. Rallis
     Title: Executive Vice President,        Title: President and
            General Counsel and                     Chief Operating Officer
            Secretary


ATTEST:                                 AMERICAN STOCK TRANSFER & TRUST
                                        COMPANY

By:         /s/ Barry S. Rosenthal      By:         /s/ Herbert J. Lemmer
     --------------------------------        -----------------------------------
     Name:  Barry S. Rosenthal               Name:  Herbert J. Lemmer
     Title: Vice President                   Title: Vice President


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