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 13, 2018

Tetraphase Pharmaceuticals, Inc.

(Exact name of registrant as specified in charter)

Registrant’s telephone number, including area code: (617) 715-3600

Not Applicable

(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):

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).

Emerging growth company ☐

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. ☐

On February 13. 2018, Tetraphase Pharmaceuticals, Inc. issued a press release announcing that the Phase 3 clinical trial of its antibiotic candidate eravacycline to treat complicated urinary tract infections did not achieve its co-primary endpoints of statistical non-inferiority compared to ertapenem. A copy of the press release announcing this event has been filed as Exhibit 99.1 to this Current Report on Form 8-K and is incorporated herein by reference.

(d) Exhibits

See Exhibit Index attached hereto.

EXHIBIT INDEX

SIGNATURE

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.