EXHIBIT 5.1

[Cooley Godward LLP Letterhead]

January 19, 2001

Inhale Therapeutic Systems, Inc.
150 Industrial Road
San Carlos, CA

Ladies and Gentlemen:

    You have requested our opinion with respect to certain matters in connection
with the filing by Inhale Therapeutic Systems, Inc. (the "Company") of a
Registration Statement on Form S-3 on or about January 19, 2001 (the
"Registration Statement") with the Securities and Exchange Commission covering
the offering of up to 3,752,456 shares of the Company's Common Stock, $.0001 par
value (the "Shares").

    In connection with this opinion, we have examined the Registration Statement
and related Prospectus, your Certificate of Incorporation and Bylaws, 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
Registration Statement and related Prospectus, will be validly issued, fully
paid, and nonassessable.

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

Very truly yours,

Cooley Godward LLP

/s/ Mark P. Tanoury

Mark P. Tanoury