POWER OF ATTORNEY
The undersigned, as a Section 16 reporting person of Pharsight
Corporation (the "Company"), hereby constitutes and appoints
William Frederick and Quynh Trinh, and each of them, 
the undersigned's true and lawful attorney-in-fact to:
1. complete and execute Forms 3, 4, and 5 and other forms and 
all amendments thereto as such attorney-in-fact shall in his or her 
discretion determine to be required or advisable
pursuant to Section 16 of the Securities  
Exchange Act of 1934 (as amended) and the rules and regulations 
promulgated thereunder, or any successor laws and regulations, as   
consequence of the undersigned's ownership, acquisition or disposition  
of securities of the Company; and
2. do all acts necessary in order to file such forms with the Securities 
and Exchange Commission, any securities exchange or national association,
the Company and such other person or agency as the attorney-in-fact shall 
deem appropriate.
   The undersigned hereby ratifies and confirms all that said 
attorneys-in-fact and agents shall do or cause to be done by virtue hereof.
The undersigned acknowledges that the foregoing attorneys-in-fact, in 
serving in such capacity at the request of the undersigned, are not 
assuming, nor is the Company assuming, any of the undersigned's 
responsibilities to comply with the Securities Exchange Act of 1934
(as amended).
   This Power of Attorney shall remain in full force and effect until
the undersigned is no longer required to file Forms 3, 4, and 5
with respect to the undersigned's holdings of and transactions in securities
issued by the Company, unless earlier revoked by the undersigned in a 
signed writing delivered to the Company and the foregoing attorneys-in-fact.
   IN WITNESS WHEREOF, the undersigned has caused this Power of Attorney 
to be executed as of this August 8, 2006.

Signature:        /s/ Arthur Reidel                        
        Arthur Reidel