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): February 29, 2016 (February 26, 2016)

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 8 — Other Events

On February 26, 2016, Alexza Pharmaceuticals, Inc. (“Alexza” or the “Company”), issued a press release announcing that on February 15, 2016, it entered into a non-binding Letter of Intent (the “Letter of Intent”) with Grupo Ferrer Internacional, S.A. (“Ferrer”) with respect to Ferrer’s proposed acquisition of all outstanding shares of the Company’s common stock (the “Transaction”). A copy of the press release is furnished as Exhibit 99.1 to this Current Report on Form 8-K and is incorporated herein by reference.

The Letter of Intent does not constitute a binding agreement to consummate such acquisition and it entitles both Alexza and Ferrer to terminate discussions at any time in their sole discretion. Additionally Alexza can, at its discretion, enter into discussions with third parties and continue to explore strategic options. There can be no assurance that such potential Transaction will be agreed to or consummated.

The entering into the Letter of Intent follows exploration of strategic options that the Company announced previously. On September 28, 2015, the Company announced that it had retained Guggenheim Securities, LLC to assist in exploring strategic options to enhance stockholder value, including a possible sale or disposition of one or more corporate assets, a strategic business combination, partnership or other transactions.

Alexza’s Board of Directors will review and carefully evaluate the terms of the potential Transaction with its financial and legal advisors. Neither the Board nor the Company intends to comment further except to the extent required by law.

This Current Report on Form 8-K contains forward-looking statements that involve significant risks and uncertainties. Any statement describing the Company’s expectations or beliefs is a forward-looking statement, as defined in the Private Securities Litigation Reform Act of 1995, and should be considered an at-risk statement. Such statements are subject to certain risks and uncertainties, particularly those inherent in the timing of and whether the Company and Ferrer may reach a final agreement on a potential acquisition. The Company’s forward-looking statements also involve assumptions that, if they prove incorrect, would cause its results to differ materially from those expressed or implied by such forward-looking statements. These and other risks concerning Alexza’s business are described in additional detail in the Company’s Annual Report on Form 10-K for the year ended December 31, 2014 and the Company’s other Periodic and Current Reports filed with the Securities and Exchange Commission. Forward-looking statements contained in this announcement are made as of this date, and the Company undertakes no obligation to publicly update any forward-looking statement, whether as a result of new information, future events or otherwise.

Section 9 — Financial Statements and Exhibits

(d) 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