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): July 1, 2015

KYTHERA BIOPHARMACEUTICALS, INC.

(Exact Name of Registrant as Specified in Charter)

30930 Russell Ranch Road, Third Floor

Westlake Village, CA 91362

(Address of Principal Executive Offices) (Zip Code)

Registrant’s telephone number, including area code: (818) 587-4500

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:

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

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

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

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

Item 1.01.                                        Entry into a Material Definitive Agreement.

Amendment to Merger Agreement

On July 1, 2015, KYTHERA Biopharmaceuticals, Inc., a Delaware corporation (the “Company”), entered into Amendment No. 1 (the “Amendment”) to the Agreement and Plan of Merger (the “Merger Agreement”) with Allergan plc, a company incorporated under the laws of Ireland (“Allergan”) and Keto Merger Sub, Inc., a Delaware corporation and wholly-owned subsidiary of Allergan (“Merger Sub”), providing for the merger of Merger Sub with and into the Company, with the Company surviving the merger as a wholly-owned subsidiary of Allergan. The purpose of the Amendment is to make a technical correction to the definition of “VWAP of Parent Stock” contained in the Merger Agreement. The full text of the Amendment, attached hereto as Exhibit 2.1, is incorporated herein by reference.

Important Information for Investors and Shareholders

This communication does not constitute an offer to sell or the solicitation of an offer to buy any securities or a solicitation of any vote or approval, nor shall there be any sale of securities in any jurisdiction in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such jurisdiction.  In connection with the proposed acquisition by Allergan of the Company, Allergan will file with the SEC a registration statement on Form S-4 that will include a proxy statement of the Company that also constitutes a prospectus of Allergan.  The definitive proxy statement/prospectus will be delivered to stockholders of the Company.  INVESTORS AND SECURITY HOLDERS OF THE COMPANY ARE URGED TO READ THE DEFINITIVE PROXY STATEMENT/PROSPECTUS AND OTHER DOCUMENTS THAT WILL BE FILED WITH THE SEC CAREFULLY AND IN THEIR ENTIRETY WHEN THEY BECOME AVAILABLE BECAUSE THEY WILL CONTAIN IMPORTANT INFORMATION.  Investors and security holders will be able to obtain free copies of the registration statement and the definitive proxy statement/prospectus (when available) and other documents filed with the SEC by Allergan and the Company through the website maintained by the SEC at www.sec.gov.  Copies of the documents filed with the SEC by Allergan will be available free of charge on Allergan’s internet website at http://www.allergan.com or by contacting Allergan’s Investor Relations Department at (862) 261-7488.  Copies of the documents filed with the SEC by the Company will be available free of charge on the Company’s internet website at http://www.kythera.com or by contacting the Company’s Investor Relations Department at (818) 587-4559.

Participants in the Merger Solicitation

Allergan, the Company, their respective directors and certain of their executive officers and employees may be considered participants in the solicitation of proxies in connection with the proposed transaction.  Information regarding the persons who may, under the rules of the SEC, be deemed participants in the solicitation of the Company stockholders in connection with the proposed merger will be set forth in the proxy statement/prospectus when it is filed with the SEC.  Information about the directors and executive officers of the Company is set forth in its

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proxy statement for its 2015 annual meeting of stockholders, which was filed with the SEC on April 23, 2015 and certain of its Current Reports on Form 8-K.  Information about the directors and executive officers of Allergan is set forth in Allergan’s proxy statement for its 2015 annual meeting of shareholders, which was filed with the SEC on April 24, 2015 and certain of its Current Reports on Form 8-K (such proxy statement and periodic public filings having been filed under the “Actavis plc” name).  Additional information regarding the participants in the proxy solicitations and a description of their direct and indirect interests, by security holdings or otherwise, will be contained in the proxy statement/prospectus filed with the above-referenced registration statement on Form S-4 and other relevant materials to be filed with the SEC when they become available.

Cautionary Statement Regarding Forward-Looking Statements

This communication contains “forward-looking statements” relating to the acquisition of the Company by Allergan. Such forward-looking statements are based on current expectations and involve inherent risks and uncertainties, including factors that could delay, divert or change any of them, and could cause actual outcomes and results to differ materially from current expectations. No forward-looking statement can be guaranteed. Among other risks, there can be no guarantee that the transaction will be completed, or if it is completed, that it will close within the anticipated time period. Forward-looking statements in the press release should be evaluated together with the many uncertainties that affect Allergan’s business, particularly those identified in the cautionary factors discussion in Allergan’s periodic public filings with the SEC, including but not limited to its Annual Report on Form 10-K for the year ended December 31, 2014 and Quarterly Report on Form 10-Q for the quarterly period ended March 31, 2015.

Except for the historical information presented herein, matters discussed herein may constitute forward-looking statements that are subject to certain risks and uncertainties that could cause actual results to differ materially from any future results, performance or achievements expressed or implied by such statements. Statements that are not historical facts, including statements preceded by, followed by, or that include the words “future”; “anticipate”; “potential”; “believe”; or similar statements are forward-looking statements. Risks and uncertainties include uncertainties as to the timing of the Merger; the possibility that various closing conditions for the transaction may not be satisfied or waived, including that a governmental entity may prohibit, delay or refuse to grant approval for the consummation of the transaction; the effects of disruption from the transaction making it more difficult to maintain relationships with employees, licensees, other business partners or governmental entities; the ability of KYBELLA™ to be a first-in-class submental contouring injectable drug; anticipated commercial availability of KYBELLA™ in June 2015; the ability of KYBELLA™ to be a less-invasive, non-surgical option for the treatment of submental fullness; expectations regarding the Company’s longer-term strategy; as well as risks detailed from time to time in the Company’s periodic public filings with the SEC, including but not limited to its Annual Report on Form 10-K for the year ended December 31, 2014 and Quarterly Report on Form 10-Q for the quarterly period ended March 31, 2015. Such forward-looking statements involve substantial risks and uncertainties that could cause the Company’s clinical development programs, future results, performance or achievements to differ significantly from those expressed or implied by the forward-looking statements. Such risks and uncertainties include, among others, the uncertainties

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inherent in the clinical drug development process, including the regulatory approval process, the Company’s substantial dependence on KYBELLA™, and other matters that could affect the availability or commercial potential of the Company’s drug candidate. The forward-looking statements made herein speak only as of the date hereof.  The Company undertakes no obligation to update or revise any forward-looking statements.

Item 9.01.                                  Financial Statements and Exhibits

(d) Exhibits.

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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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EXHIBIT INDEX

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