UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the

Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): April 29, 2015 (April 28, 2015)

ALEXZA PHARMACEUTICALS, INC.

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (650) 944-7000

(Former name or former address, if changed since last report.)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (see General Instruction A.2. below):

Section 5 — Corporate Governance and Management

Item 5.02. Departure of Directors or Certain Officers; Election of Directors; Appointment of Certain Officers; Compensatory Arrangements of Certain Officers.

(c) On April 28, 2015, the Board of Directors (the “Board”) of Alexza Pharmaceuticals, Inc., a Delaware corporation (the “Company”) promoted Dr. Edwin S. Kamemoto, Ph.D., age 61, to the newly created position of Executive Vice President, R&D and Quality. A copy of the press release relating to Dr. Kamemoto’s appointment is attached hereto as Exhibit 99.1.

Dr. Kamemoto will receive a base annual salary of $350,000. In connection with the foregoing promotion, the compensation committee of the Board granted Dr. Kamemoto a stock option to purchase 35,000 shares of the Company’s common stock (the “Option”). The Option was granted under and in accordance with the terms and conditions of the Company’s 2005 Equity Incentive Plan (the “Plan”) and the Form of Notice of Grant of Award, Form of Option Agreement and Form of Stock Unit Award Agreement (the “Related Agreements”) previously filed with the SEC as Exhibit 10.5 to the Company’s registration statement on Form S-1 on December 22, 2005.

Pursuant to the Plan and the Related Agreements, the Option will vest as to 25% of the underlying shares of common stock on April 28, 2016 and will vest as to the remaining underlying shares monthly over the following 36 months, provided in each case that Dr. Kamemoto remains employed by the Company through each applicable vesting date. The exercise price for the Option is $1.95, the closing price for the Company’s Common Stock on the Nasdaq Capital Market on the date of such grant.

The foregoing summary of the Related Agreements is qualified in its entirety by reference to the full text of the Related Agreements that the Company has previously filed with the SEC.

Section 9 — Financial Statements and Exhibits

Item 9.01. Financial Statements and Exhibits.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

INDEX TO EXHIBITS