Exhibit 5.1

    [Cooley Letterhead]

May 3, 2001

Pharsight Corporation
800 West El Camino Real, Suite 200
Mountain View, California 94040

Ladies and Gentlemen:

    You have requested our opinion with respect to certain matters in connection with the filing by Pharsight Corporation (the "Company") of a Registration Statement on Form S-8 (the "Registration Statement") with the Securities and Exchange Commission covering the offering of up to 330,000 shares of the Company's Common Stock, $.001 par value, (the "Shares") pursuant to its 2001 UK Employee Stock Purchase Plan and UK Company Share Option Plan (together the "Plans").

    In connection with this opinion, we have examined the Registration Statement, your Restated and Amended Certificate of Incorporation and By-laws, as amended, and such other documents, records, certificates, memoranda and other instruments as we deem necessary as a basis for this opinion. We have assumed the genuineness and authenticity of all documents submitted to us as originals, the conformity to originals of all documents submitted to us as copies thereof, and the due execution and delivery of all documents where due execution and delivery are a prerequisite to the effectiveness thereof.

    On the basis of the foregoing, and in reliance thereon, we are of the opinion that the Shares, when sold and issued in accordance with the Plans, the Registration Statement and related Prospectus, will be validly issued, fully paid, and nonassessable (except as to shares issued pursuant to certain deferred payment arrangements, which will be fully paid and nonassessable when such deferred payments are made in full).

    We consent to the filing of this opinion as an exhibit to the Registration Statement.

Very truly yours,

COOLEY GODWARD LLP

By: /s/ BRETT D. WHITE Brett D. White