Exhibit 10.47  May 22, 2003  Alloy Ventures 480 Cowper Street Second Floor Palo Alto, California Attn: 
Tony Di Bona  Sprout Group 11 Madison Avenue 13th Floor New York, NY 10010 Attn: Kathleen D. La Porte  Re:          Waiver of Filing Post-Effective
Amendment on Form S-1 Ladies and Gentlemen: Reference is hereby made to that certain Preferred Stock and Warrant
Purchase Agreement dated as of June 25, 2002, as amended on February 13, 2003,
(the Purchase Agreement) by and among Pharsight Corporation (Pharsight) and
each of the purchasers named therein (collectively, the Purchasers).  All capitalized but undefined terms used
herein shall have the meanings ascribed to them in the Purchase Agreement. Pursuant to Section 9.2(b) of the Purchase Agreement, Pharsight hereby
notifies each of the Purchasers that, effective upon the filing of Pharsights
Annual Report on Form 10-K for Pharsights fiscal year ended March 31, 2003,
the Initial Registration Statement shall cease to be effective for the resale
of the Registrable Securities as Pharsight shall cease to be eligible to
register the resale of the Registrable Securities on Form S-3.   Consequently, Pharsight is obligated to use
its commercially reasonable efforts to file a post-effective amendment on Form
S-1 and cause such post-effective amendment to become effective within 90 days
after the delivery notice of this Suspension Notice. Pharsight hereby requests each of the Purchasers to waive the
requirement under the Purchase Agreement that Pharsight file a post-effective
amendment on Form S-1 when and for so long as Pharsight ceases to be eligible
to register the resale of the Registrable Securities on Form S-3.  Further, Pharsight hereby requests each of
the Purchasers to waive any rights to any Liquidated Damages it may have
pursuant to Section 9.2(f) of the Purchase Agreement as a result of the failure
to file a post-effective amendment on Form S-1.  In this regard, each of the Purchasers hereby acknowledges that
if this waiver is given: (a) Pharsight will rely on this waiver in connection
with its determination not to file a post-effective amendment on Form S-1; and
(b) Pharsight shall not be obligated to make any payment of Liquidated Damages
to any Purchaser for failure to cause the use of the suspended Prospectus to be
resumed during the period during which this waiver is in effect. Notwithstanding the foregoing, in the event that any Purchaser shall
notify Pharsight that it is  
   terminating this waiver (the Waiver Termination Notice), then, in
such case: (a) the waiver set forth herein shall cease to be effective from
that day forward; (b) Pharsight shall file such post-effective amendment on
Form S-1, as set forth in the Purchase Agreement, within thirty (30) days of
receipt of such Waiver Termination; (c) for all purposes of the Purchase
Agreement, including without limitation the liquidated damages provisions
thereof, Pharsights obligations with respect to the filing of such
post-effective amendment on Form S-1 shall resume as if Pharsight had first given
the Suspension Notice on the date that Pharsight receives the Waiver
Termination; and (d) no Registration Default shall have been deemed to occur
unless and until the earlier of (x) failure by Pharsight to file such
post-effective amendment on Form S-1 within thirty (30) days of delivery of the
Waiver Termination Notice and (y) failure of such post-effective amendment on
Form S-1 to become effective within ninety (90) days of delivery of the Waiver
Termination Notice. Pharsight covenants and agrees (a) to use its commercially reasonable
efforts to become eligible to register the resale of the Registrable Securities
on Form S-3 as promptly as practicable after the date hereof, and (b) except to
the extent contractually obligated pursuant to contracts existing on the date
hereof, not to file a registration statement with respect to any securities of
Pharsight, other than a registration statement on Form S-8 and the
post-effective amendment on Form S-1, without the prior written consent of
Purchasers who hold at last 75% of the Registrable Securities. In the event that Purchasers who hold at least 75% of the Registrable
Securities countersign this waiver, then, in accordance with Section 9.11 of
the Agreement, this waiver will be binding against all Purchasers. All other provisions of the Purchase Agreement shall remain in full
force and effect. Please
indicate your waiver with respect to such rights by countersigning and
returning this Waiver Letter as soon as possible. Sincerely, PHARSIGHT
CORPORATION  /s/  Shawn M. OConnor  Shawn M. OConnor President and Chief
Executive Officer   
   The
undersigned have read the foregoing and hereby agree to be bound by its
terms.  This Waiver Letter may be
countersigned in several counterparts, each of which shall constitute an
original and all of which, when taken together, shall constitute one
instrument.  PURCHASER(S): ALLOY PARTNERS 2000, L.P. ALLOY VENTURES 2000, L.P. ALLOY CORPORATE 2000, L.P. ALLOY INVESTORS 2000, L.P.  By:
  Alloy
  Ventures 2000, LLC, its General PartnerBy:
  /s/  Douglas E. KellyName:Title:  Managing MemberAddress:
  480 Cowper StreetSecondFloorPalo Alto, CaliforniaAttn:  Tony
  Di BonaEmail:
  cfo@alloyventures.comFacsimile:
  (650) 687-5010  
  PURCHASER(S) CONTINUED: DONALDSON, LUFKIN & JENRETTE SECURITIES
CORPORATION, as nominee for DLJ First ESC, L.P., EMA 2001 Plan, L.P., CSFB 2001
Investors, L.P., Credit Suisse First Boston Private Equity, Inc., Docklands
2001 Plan, L.P., and Paradeplatz 2001 Plan, L.P.  By:
  /s/  Philippe ChambonName:  Philippe ChambonIts: Attorney-in-FactAddress:
  Sprout Group11 Madison Avenue13th FloorNew York, NY 10010Email:Facsimile:
  (646) 935-7094  SPROUT ENTREPRENEURS FUND, L.P.  By:
  DLJ
  Capital Corp., its General PartnerBy:
  /s/  Philippe ChambonName:  Philippe ChambonIts:  Managing DirectorAddress:
  Sprout Group11 Madison Avenue13th FloorNew York, NY 10010Email:Facsimile:
  (646) 935-7094   
  PURCHASER(S) CONTINUED: SPROUT CAPITAL IX, L.P.  By:
  DLJ
  Capital Corp., its Managing General PartnerBy:
  /s/  Philippe ChambonName:  Philippe ChambonIts:  Managing DirectorAddress:
  Sprout Group11 Madison Avenue13th FloorNew York, NY 10010Email:Facsimile:
  (646) 935-7094  SPROUT CAPITAL VII, L.P.  By:
  DLJ
  Capital Corp., its Managing General PartnerBy:
  /s/  Philippe ChambonName:  Philippe ChambonIts:  Managing DirectorAddress:
  Sprout Group11 Madison Avenue13th FloorNew York, NY 10010Email:Facsimile:
  (646) 935-7094   
  PURCHASER(S) CONTINUED: SPROUT CEO FUND, L.P.  By:
  DLJ
  Capital Corp., its Managing General PartnerBy:
  /s/  Philippe ChambonName:  Philippe ChambonIts:  Managing DirectorAddress:
  Sprout Group11 Madison Avenue13th FloorNew York, NY 10010Email:Facsimile:
  (646) 935-7094  DLJ CAPITAL CORP.  By:
  /s/  Philippe ChambonName:  Philippe ChambonIts:  Managing DirectorAddress:
  Sprout Group11 Madison Avenue13th FloorNew York, NY 10010Email:Facsimile:
  (646) 935-7094  DLJ FIRST ESC L.P.  By:
  DLJ
  LBO Plans Management Corporation, its General PartnerBy:
  /s/  Philippe ChambonName:  Philippe ChambonIts:  Attorney-in-FactAddress:
  Sprout Group11 Madison Avenue13th FloorNew York, NY 10010Email:Facsimile:
  (646) 935-7094