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                                                                   EXHIBIT 10.13

                    FOURTH AMENDMENT TO CONSULTING AGREEMENT

THIS FOURTH AMENDMENT TO Consulting agreement ("FOURTH AMENDMENT") is entered
into and made effective as of December 31, 2004 ("FOURTH AMENDMENT EFFECTIVE
DATE") by and between Connetics Corporation ("CONNETICS") and Eugene A. Bauer,
M.D. ("DR. BAUER"). Unless otherwise indicated, a capitalized term within this
Fourth Amendment shall have the same meaning as the same capitalized term within
the Initial Agreement (defined below).

                                    AGREEMENT

Connetics and Dr. Bauer are Parties to a Consulting Agreement dated January 1,
2002 ("INITIAL AGREEMENT"), to an Amendment to Consulting Agreement dated
December 11, 2002 ("AMENDMENT"), to a Second Amendment to Consulting Agreement
dated January 1, 2003 ("SECOND AMENDMENT") and to a Third Amendment to
Consulting Agreement dated December 31, 2003 ("THIRD AMENDMENT"). The Initial
Agreement, the Amendment, the Second Amendment and the Third Amendment are to be
known collectively as the "CONSULTING AGREEMENT." Connetics and Dr. Bauer desire
to amend the terms of the Consulting Agreement as set forth below.

1.   Section 2(a) of the Consulting Agreement is superseded and replaced, in its
     entirety, to read as follows:

          2(a) Connetics hereby retains Dr. Bauer and Dr. Bauer hereby accepts
               this retainer as a consultant to Connetics for the term
               commencing on January 1, 2002 and ending on December 31, 2005.

2.   Section 5(a) of the Consulting Agreement is superseded and replaced, in its
     entirety, to read as follows:

          5(a) Annual Retainer. Beginning on the date of the Annual Meeting of
               Shareholders of Connetics to be held in approximately April 2005
               ("Annual Meeting") and continuing throughout the term of the
               Consulting Agreement, Connetics shall pay Dr. Bauer, and Dr.
               Bauer shall accept for Dr. Bauer's services under this Consulting
               Agreement, compensation in the form of an annual retainer in the
               amount of Thirty-Six Thousand Dollars ($36,000) ("New Retainer")
               payable in quarterly installments of Nine Thousand Dollars
               ($9,000); provided, however, for the calendar quarter commencing
               April 1, 2005 the quarterly installment payable to Dr. Bauer
               shall be a pro-rated amount equal to (a) the annual retainer in
               effect for the period prior to the Annual Meeting and (b) the New
               Retainer from the date of the Annual Meeting through the end of
               the calendar quarter, with such amounts pro-rated based on the
               actual number of days in the applicable period.

3.   This Fourth Amendment may be executed in a number of identical
     counterparts, each of which will be deemed an original, but all of which
     together will constitute one and the same instrument.

4.   All of the remaining provisions of the Consulting Agreement shall continue
     in full force and effect as though fully set forth in this Fourth
     Amendment.

IN WITNESS WHEREOF, the Parties have entered into this Fourth Amendment as of
the Fourth Amendment Effective Date.

     "CONNETICS"                                    "DR. BAUER"
  CONNETICS CORPORATION                           EUGENE A. BAUER



Signature: /s/ Thomas G. Wiggans          Signature: /s/ Eugene A. Bauer
           ------------------------                  -----------------------
Print: Thomas G. Wiggans                  Print: Eugene A. Bauer, M.D.
Title: President and Chief                Social Security No:  ____________
       Executive Officer