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): August 14, 2014

QUESTCOR PHARMACEUTICALS, INC.

(Exact Name of Registrant as Specified in its Charter)

1300 Kellogg Drive, Suite D

Anaheim, California 92807

(Address of Principal Executive Offices, including Zip code)

(714) 786-4200

(Registrant’s telephone number, including area code)

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:

On August 14, 2014, pursuant to the Agreement and Plan of Merger (the “Merger Agreement”), dated as of April 5, 2014, among Questcor Pharmaceuticals, Inc. (the “Company”), Mallinckrodt plc (“Mallinckrodt”) and Quincy Merger Sub, Inc. (“Merger Sub”), Merger Sub merged with and into the Company, with the Company being the surviving entity (the “Merger”). As a result of the Merger, the Company became a wholly owned indirect subsidiary of Mallinckrodt.

As previously disclosed, under the Merger Agreement, each holder of a share of Company common stock, no par value, issued and outstanding immediately prior to the Merger (other than dissenting shares, shares of restricted Company common stock granted to individuals other than non-employee directors, and Company common stock owned by the Company, Mallinckrodt, Merger Sub or any of their respective subsidiaries) had the right to receive a combination of (1) $30.00 per share of Company common stock in cash, without interest (the “Cash Consideration”), plus (2) 0.897 of a Mallinckrodt ordinary share (the “Stock Consideration,” and together with the Cash Consideration, the “Merger Consideration”).

The issuance of Mallinckrodt ordinary shares in connection with the Merger was registered under the Securities Act of 1933, as amended, pursuant to Mallinckrodt’s registration statement on Form S-4 (File No. 333-196054) (the “Registration Statement”) filed with the Securities and Exchange Commission (the “SEC”) and declared effective July 11, 2014. The definitive joint proxy statement/prospectus of the Company and Mallinckrodt that forms a part of the Registration Statement (the “Joint Proxy Statement/Prospectus”) contains additional information about the Merger and the other transactions contemplated by the Merger Agreement, including information concerning the interests of directors, executive officers and affiliates of the Company and Mallinckrodt in the Merger.

The foregoing description of the Merger Agreement and the Merger is not complete and is qualified in its entirety by reference to the Merger Agreement, which was included as Exhibit 2.1 to the Company’s Current Report on Form 8-K, filed with the SEC on April 7, 2014, which is incorporated by reference herein.

On August 14, 2014, in connection with the Merger, the Company notified the NASDAQ Stock Market (“NASDAQ”) that the Merger had been completed and requested that trading of Company common stock on NASDAQ be suspended after the close of trading on August 14, 2014. In addition, the Company requested that NASDAQ file with the SEC a notification of removal from listing on Form 25 to delist Company common stock from NASDAQ and deregister Company common stock under Section 12(b) of the Securities Exchange Act of 1934, as amended (the “Exchange Act”). On August 14, 2014, in accordance with the Company’s request, NASDAQ filed the Form 25 with the SEC. The Company intends to file a certification and notice of termination on Form 15 requesting that the Company’s reporting obligations under Sections 13 and 15(d) of the Exchange Act be terminated. The information set forth in Item 2.01 is incorporated by reference into this Item 3.01.

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In connection with the Merger, on August 14, 2014 each share of Company common stock was cancelled and automatically converted into the right to receive the Merger Consideration.

Immediately prior to the effective time of the Merger (the “Effective Time”):

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The information set forth in Items 2.01 and 3.01 is incorporated by reference into this Item 3.03.

As a result of the Merger, a change of control of the Company occurred and the Company became a wholly owned subsidiary of Mallinckrodt. The information set forth in Items 2.01 and 3.01 is incorporated by reference into this Item 5.01.

At the Effective Time, each member of the Company’s board of directors ceased to be a director of the Company. The members of the Company’s board of directors immediately prior to the Merger were Virgil D. Thompson, Don M. Bailey, Neal C. Bradsher, Stephen C. Farrell, G. Kelly Martin, Angus C. Russell, Louis E. Silverman and Scott M. Whitcup, M.D.

Immediately after the Effective Time, Messrs. Bailey, Russell and Thompson were appointed to the board of directors of Mallinckrodt pursuant to the terms of the Merger Agreement, which provides that Mallinckrodt will take such actions as are necessary to appoint such individuals to the Mallinckrodt board of directors. In addition, immediately after the Effective Time, Don Bailey resigned as President and Chief Executive Officer of the Company.

In accordance with the terms of the Merger Agreement, the directors of Merger Sub immediately prior to the Effective Time, as set forth below, became the directors of the Company until the earlier of their resignation or removal or until their respective successors are duly designated, as the case may be. In addition, after the Effective Time, the officers set forth below became the officers of the Company until the earlier of their resignation or removal or until their respective successors are duly designated, as the case may be.

Biographical and other information with respect to the new directors of the Company is set forth below:

Kathleen A. Schaefer has served as Vice President, Finance and Corporate Controller of Mallinckrodt since June 2013. She has also served as the President and a director of Merger Sub since April 2014. Ms. Schaefer previously served as controller of the Pharmaceuticals business of Covidien plc (“Covidien”) from June 2007 until Mallinckrodt’s separation from Covidien in June 2013.

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Marvin R. Haselhorst has served as Vice President, Tax of Mallinckrodt since June 2013. Mr. Haselhorst has also served as Vice President and a director of Merger Sub since April 2014. Mr. Haselhorst previously served as head of the tax function for Covidien’s Pharmaceuticals business from July 2012 until its separation from Covidien in June 2013. From June 2006 until June 2012, Mr. Haselhorst was Vice President, Global Tax for Solutia, Inc., a specialty chemical manufacturer and distributor headquartered in St. Louis, Missouri.

John E. Einwalter has served as Vice President and Treasurer of Mallinckrodt since June 2013. Mr. Einwalter also has served as Vice President, Treasurer and a director of Merger Sub since April 2014. Mr. Einwalter previously served as treasurer of Covidien’s Pharmaceuticals business from October 2012 until its separation from Covidien in June 2013. Previously, Mr. Einwalter was Vice President and Treasurer of Belden Inc., a global manufacturer of connectivity and networking equipment, from July 2011. He was the Director of Treasury at Smurfit-Stone Container Corporation, a paperboard and paper-based packaging company, from February 2010 to July 2011. From September 2003 to January 2010, Mr. Einwalter served in various finance positions with Anheuser-Busch InBev.

On August 14, 2014, the Company held a special meeting of shareholders (the “Special Meeting”) to consider certain proposals related to the Merger Agreement, which provides, among other things and subject to the terms and conditions set forth therein, that the Company will be merged with and into Merger Sub with the Company as the surviving entity.

As of July 9, 2014, the record date for the Special Meeting, there were 61,420,933 shares of Company common stock outstanding, each of which was entitled to one vote for each proposal at the Special Meeting. At the Special Meeting, a total of 46,482,915 shares of Company common stock, representing approximately 76% of outstanding shares entitled to vote, were present in person or by proxy, constituting a quorum to conduct business.

At the Special Meeting, the following proposals were considered:

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All of the proposals were approved by the requisite vote of the Company’s shareholders. The final voting results for each proposal are described below. For more information on each of these proposals, see the Joint Proxy Statement/Prospectus for the Special Meeting, filed with the SEC on July 14, 2014.

Although Proposal 3 was approved, adjournment of the Special Meeting was not necessary or appropriate because there were sufficient votes at the time of the Special Meeting to approve and adopt the Merger Agreement.

On August 14, 2014, the Company and Mallinckrodt issued a joint press release announcing (1) the results of the Special Meeting and (2) the results of the extraordinary general meeting of shareholders of Mallinckrodt held on August 14, 2014. A copy of the press release is attached hereto as Exhibit 99.1 and is incorporated by reference herein.

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SIGNATURE

Pursuant to the requirements of Section 12 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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