SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, DC 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):  November 26, 2013

CUBIST PHARMACEUTICALS, INC.

(Exact Name of Registrant as Specified in Charter)

Registrant’s telephone number, including area code:        (781) 860-8660

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:

o Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

o Soliciting material pursuant to Rule 14a-12(b) under the Exchange Act (17 CFR 240.14a-12(b))

o Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

o Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Item 8.01  Other Events.

Net product sales of DIFICID® (fidaxomicin) in the U.S. and Canada by Optimer Pharmaceuticals, Inc., or Optimer, were $15.5 million for the quarter ended September 30, 2013, and were negatively impacted by a $1.6 million adjustment to revenue reserves recorded during the period due to higher returns of DIFICID than estimated since its launch in July 2011.  Optimer became a wholly owned subsidiary of the registrant, Cubist Pharmaceuticals, Inc., as a result of the registrant completing its acquisition of Optimer on October 24, 2013.

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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.

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