Prepared by MERRILL CORPORATION
QuickLinks -- Click here to rapidly navigate through this document

As filed with the Securities and Exchange Commission on February 8, 2002



SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549


SCHEDULE TO
(Amendment No. 1)
Tender Offer Statement under Section 14(d)(1) or 13(e)(1)
of the Securities Exchange Act of 1934


Inhale Therapeutic Systems, Inc.
(Name of Subject Company—Issuer and Filing Person—Offeror)


OPTIONS TO PURCHASE COMMON STOCK,
par value $0.0001 per share
(Title of Class of Securities)


457191-10-4
(CUSIP Number of Class of Securities)


Ajit S. Gill
Chief Executive Officer and President
INHALE THERAPEUTIC SYSTEMS, INC.
150 Industrial Road
San Carlos, California 94070
Telephone: (650) 631-3100
(Name, address and telephone number of person authorized to receive
notices and communications on behalf of Filing Person)


Copies to:
Mark P. Tanoury
Thomas Z. Reicher
John M. Geschke
COOLEY GODWARD LLP
3175 Hanover Street
Palo Alto, California 94304-1130
Telephone: (650) 843-5000


CALCULATION OF FILING FEE


Transaction Valuation*   Amount of Filing Fee**

$33,906,010.70   $3,119.35
*
Calculated solely for purposes of determining the filing fee. This amount assumes that options to purchase 3,653,680 shares of Common Stock of Inhale Therapeutic Systems, Inc. having an aggregate value of $33,906,010.70 will be exchanged pursuant to this offer. The aggregate value of such options was calculated based on the Black-Scholes option pricing model as of January 23, 2002.

**
$92 per $1,000,000 of the aggregate offering amount (or .000092 of the aggregate transaction valuation), pursuant to Rule 0-11 of the Securities Exchange Act of 1934, as amended by Fee Advisory #8, effective January 16, 2002. The filing fee was previously paid with the Schedule TO filing made with the Securities and Exchange Commission on January 25, 2002.
    Amount Previously Paid:   Not applicable. Filing Party:   Not applicable.
    Form or Registration No.:   Not applicable. Date Filed:   Not applicable.

Check the appropriate boxes below to designate any transactions to which the statement relates:

    / /   Third-party tender offer subject to Rule 14d-1.
    /x/   Issuer tender offer subject to Rule 13e-4.
    / /   Going-private transaction subject to Rule 13e-3.
    / /   Amendment to Schedule 13D under Rule 13d-2.



CUSIP NO. 457191-10-4


SCHEDULE TO

INTRODUCTORY STATEMENT

        This Amendment No. 1 to Schedule TO amends and supplements the Tender Offer Statement on Schedule TO filed with the Securities and Exchange Commission on January 25, 2002, in connection with our offer to exchange certain options granted prior to July 24, 2001 with exercise prices per share greater than or equal to $25.00 per share currently outstanding under the Company's 2000 Non-Officer Equity Incentive Plan, as amended (the "Option Plan"), to purchase shares of the Company's Common Stock, par value $0.0001 per share (the "Common Stock"), held by optionholders as of January 25, 2002 for replacement options to purchase shares of Common Stock to be granted under the Option Plan, upon the terms and subject to the conditions set forth in the Offer to Exchange.

ITEM 12. EXHIBITS.

Exhibit Number

  Description
99.(a)(1)(A)*   Offer to Exchange, dated January 25, 2002.
99.(a)(1)(B)*   Form of Electronic Letter of Transmittal.
99.(a)(1)(C)*   Form of Summary of Terms.
99.(a)(1)(D)*   Form of Election Form.
99.(a)(1)(E)*   Form of Notice of Change in Election from Accept to Reject.
99.(a)(1)(F)*   Form of Notice of Change in Election from Reject to Accept.
99.(a)(1)(G)*   Powerpoint slide presentation to holders of Eligible Options.
99.(a)(1)(H)*   Form of Confirmation of Participation in the Offer to Exchange.
99.(a)(1)(I)   Inhale Therapeutic Systems, Inc. Annual Report on Form 10-K, as amended, for its fiscal year ended December 31, 2000, filed with the Securities and Exchange Commission on March 1, 2001 and incorporated herein by reference.
99.(a)(1)(J)   Inhale Therapeutic Systems, Inc. Current Report on Form 8-K/A, filed with the Securities and Exchange Commission on August 10, 2001, as amended, and incorporated herein by reference.
99.(a)(1)(K)   Inhale Therapeutic Systems, Inc. Current Report on Form 8-K/A, filed with the Securities and Exchange Commission on October 4, 2001 and incorporated herein by reference.
99.(a)(1)(L)   Form of Electronic Communication—Notice to Employees of Amendments to the Offer to Exchange (including Exhibit 1).
99.(a)(1)(M)   Form of Electronic Communication—Reminder to Employees of Cancellation Date.
99.(b)   Not applicable.
99.(d)(1)   Inhale Therapeutic Systems, Inc.'s 2000 Non-Officer Equity Incentive Plan, as amended, (incorporated herein by reference to the indicated exhibit in Inhale's Registration Statement on Form S-8/A (No. 333-71936), filed on January 15, 2002.
99.(g)   Not applicable.

*
Previously filed as an exhibit to the Schedule TO filed with the Securities and Exchange Commission on January 25, 2002.

CUSIP NO. 457191-10-4

SCHEDULE TO


SIGNATURE

        After due inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Dated: February 8, 2002    
  INHALE THERAPEUTIC SYSTEMS, INC.

 

By:

/s/ Ajit S. Gill

  Name: Ajit S. Gill
Title: Chief Executive Officer and President

CUSIP NO. 457191-10-4


SCHEDULE TO

INDEX OF EXHIBITS

Exhibit Number

  Description
99.(a)(1)(A)*   Offer to Exchange, dated January 25, 2002.
99.(a)(1)(B)*   Form of Electronic Letter of Transmittal.
99.(a)(1)(C)*   Form of Summary of Terms.
99.(a)(1)(D)*   Form of Election Form.
99.(a)(1)(E)*   Form of Notice of Change in Election from Accept to Reject.
99.(a)(1)(F)*   Form of Notice of Change in Election from Reject to Accept.
99.(a)(1)(G)*   Powerpoint slide presentation to holders of Eligible Options.
99.(a)(1)(H)*   Form of Confirmation of Participation in the Offer to Exchange.
99.(a)(1)(I)   Inhale Therapeutic Systems, Inc. Annual Report on Form 10-K, as amended, for its fiscal year ended December 31, 2000, filed with the Securities and Exchange Commission on March 1, 2001 and incorporated herein by reference.
99.(a)(1)(J)   Inhale Therapeutic Systems, Inc. Current Report on Form 8-K/A, filed with the Securities and Exchange Commission on August 10, 2001, as amended, and incorporated herein by reference.
99.(a)(1)(K)   Inhale Therapeutic Systems, Inc. Current Report on Form 8-K/A, filed with the Securities and Exchange Commission on October 4, 2001 and incorporated herein by reference.
99.(a)(1)(L)   Form of Electronic Communication—Notice to Employees of Amendments to the Offer to Exchange (including Exhibit 1).
99.(a)(1)(M)   Form of Electronic Communication—Reminder to Employees of Cancellation Date.
99.(b)   Not applicable.
99.(d)(1)   Inhale Therapeutic Systems, Inc.'s 2000 Non-Officer Equity Incentive Plan, as amended, (incorporated herein by reference to the indicated exhibit in Inhale's Registration Statement on Form S-8/A (No. 333-71936), filed on January 15, 2002.
99.(g)   Not applicable.

*
Previously filed as an exhibit to the Schedule TO filed with the Securities and Exchange Commission on January 25, 2002.



QuickLinks

CALCULATION OF FILING FEE
SCHEDULE TO
INTRODUCTORY STATEMENT
SIGNATURE
SCHEDULE TO