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): July 30, 2014

Mallinckrodt public limited company

(Exact name of registrant as specified in its charter)

Damastown, Mulhuddart

Dublin 15, Ireland

(Address of principal executive offices)

+353 (1) 880-8180

(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 July 30, 2014, Mallinckrodt International Finance S.A. and Mallinckrodt CB LLC (together, the “Issuers”), each of which is a wholly owned subsidiary of the Mallinckrodt plc (“Mallinckrodt” or the “Company”), entered into an agreement which will result in the private placement of $900 million of 5.75% senior unsecured notes due August 1, 2022 (the “Notes”).

Concurrently with, or in advance of, the closing of the Company’s previously announced acquisition of Questcor Pharmaceuticals, Inc. (“Questcor”), the Issuers also expect to enter into a $700 million senior secured term loan facility (the “New Term Loan”) structured as an incremental tranche under the credit agreement governing the Issuer’s existing senior secured term loan and revolving credit facility (such existing senior secured credit facilities, together with the New Term Loan, the “Senior Secured Credit Facilities”). The Issuers expect to use the proceeds from the Notes, borrowings under the New Term Loan and the initial advances under the $160 million accounts receivable securitization facility (which was entered into by certain wholly owned subsidiaries of the Company on July 28, 2014, as more fully described in the Company’s Current Report on Form 8-K filed on July 29, 2014) to fund a portion of the cash consideration payable in connection with the Questcor acquisition and to pay fees and expenses in connection with the Questcor acquisition, the entry into the facilities described above and the consummation of the Notes offering described above.

In the event that (i) the Questcor acquisition is not consummated on or before January 26, 2015, or (ii) the Agreement and Plan of Merger Agreement, dated as of April 5, 2014, by and among Mallinckrodt, Questcor and Quincy Merger Sub, Inc. is terminated in accordance with its terms at any time prior thereto, then the Issuers will be required to redeem all of the Notes at the issue price plus accrued and unpaid interest, if any, to, but excluding, the redemption date.

The Notes will initially be jointly and severally guaranteed, subject to certain exceptions, on an unsecured, unsubordinated basis by each of the Issuers’ subsidiaries that guarantees the obligations under the Issuers’ Senior Secured Credit Facilities, which subsidiaries, after the consummation of the Questcor acquisition, are expected to include Questcor and certain of its subsidiaries. Mallinckrodt will not guarantee the notes.

The Notes were offered only to qualified institutional buyers in accordance with Rule 144A under the Securities Act of 1933, as amended (the “Securities Act”), and to certain non-U.S. persons in accordance with Regulation S under the Securities Act. The Notes will not be registered under the Securities Act and may not be offered or sold without registration unless an exemption from such registration is available. This notice does not constitute an offer to sell the Notes, nor a solicitation for an offer to purchase the Notes.

Cautionary Statement Regarding Forward-Looking Statements

Statements in this communication that are not strictly historical, including statements regarding the proposed acquisition, the expected timetable for completing the transaction, future financial and operating results, benefits and synergies of the transaction, future opportunities for

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the combined businesses and any other statements regarding events or developments that we believe or anticipate will or may occur in the future, may be “forward-looking” statements within the meaning of the Private Securities Litigation Reform Act of 1995, and involve a number of risks and uncertainties. There are a number of important factors that could cause actual events to differ materially from those suggested or indicated by such forward-looking statements and you should not place undue reliance on any such forward-looking statements. These factors include risks and uncertainties related to, among other things: general economic conditions and conditions affecting the industries in which Mallinckrodt and Questcor operate; the commercial success of Mallinckrodt’s and Questcor’s products, including H.P. Acthar® Gel; Mallinckrodt’s and Questcor’s ability to protect intellectual property rights; the parties’ ability to satisfy the merger agreement conditions and consummate the merger on the anticipated timeline or at all; the availability of financing, including the financing contemplated by the debt commitment letter, on anticipated terms or at all; Mallinckrodt’s ability to successfully integrate Questcor’s operations and employees with Mallinckrodt’s existing business; the ability to realize anticipated growth, synergies and cost savings; Questcor’s performance and maintenance of important business relationships; the lack of patent protection for Acthar, and the possible U.S. Food and Drug Administration (“FDA”) approval and market introduction of additional competitive products; Questcor’s reliance on Acthar for substantially all of its net sales and profits; Questcor’s ability to continue to generate revenue from sales of Acthar to treat on-label indications associated with nephrotic syndrome, multiple sclerosis, infantile spasms or rheumatology-related conditions, and Questcor’s ability to develop other therapeutic uses for Acthar; volatility in Questcor’s Acthar shipments, estimated channel inventory, and end-user demand; an increase in the proportion of Questcor’s Acthar unit sales comprised of Medicaid-eligible patients and government entities; Questcor’s research and development risks, including risks associated with Questcor’s work in the areas of nephrotic syndrome and lupus, and Questcor’s efforts to develop and obtain FDA approval of Synacthen^TM Depot; Mallinckrodt’s ability to receive procurement and production quotas granted by the U.S. Drug Enforcement Administration; Mallinckrodt’s ability to obtain and/or timely transport molybdenum-99 to our technetium-99m generator production facilities; customer concentration; cost-containment efforts of customers, purchasing groups, third-party payors and governmental organizations; Mallinckrodt’s ability to successfully develop or commercialize new products; competition; Mallinckrodt’s ability to achieve anticipated benefits of price increases; Mallinckrodt’s ability to integrate acquisitions of technology, products and businesses generally; product liability losses and other litigation liability; the reimbursement practices of a small number of large public or private issuers; complex reporting and payment obligations under healthcare rebate programs; changes in laws and regulations; conducting business internationally; foreign exchange rates; material health, safety and environmental liabilities; litigation and violations; information technology infrastructure; and restructuring activities. Additional information regarding the factors that may cause actual results to differ materially from these forward-looking statements is available in (i) Mallinckrodt’s SEC filings, including its Annual Report on Form 10-K for the fiscal year ended September 27, 2013 and its Quarterly Reports on Form 10-Q for the quarterly periods ended December 27, 2013 and March 28, 2014; (ii) the SEC filings of Cadence Pharmaceuticals, Inc., which was acquired by Mallinckrodt on March 19, 2014, including its Annual Report on Form 10-K for the fiscal year ended December 31, 2013; and (iii) Questcor’s SEC filings, including its Annual Report on Form 10-K for the year ended December 31, 2013 (and the amendment thereto on Form 10-K/A), and its Quarterly Reports on Form 10-Q for the quarterly periods ended March 31, 2014 and June 30, 2014 and its

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Current Report on Form 8-K filed with the SEC on July 10, 2014. The forward-looking statements made herein speak only as of the date hereof and none of Mallinckrodt, Questcor or any of their respective affiliates assumes any obligation to update or revise any forward-looking statement, whether as a result of new information, future events and developments or otherwise, except as required by law.

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 transaction between Mallinckrodt and Questcor, Mallinckrodt has filed with the Securities and Exchange Commission (the “SEC”) a registration statement on Form S-4 (File No. 333-196054) containing a joint proxy statement of Mallinckrodt and Questcor that also constitutes a prospectus of Mallinckrodt. The registration statement on Form S-4 was declared effective by the SEC on July 11, 2014. Each of Mallinckrodt and Questcor commenced mailing the joint proxy statement/prospectus to its respective shareholders on July 14, 2014. INVESTORS AND SECURITY HOLDERS OF MALLINCKRODT AND QUESTCOR ARE URGED TO READ THE JOINT PROXY STATEMENT/PROSPECTUS AND OTHER DOCUMENTS THAT HAVE BEEN OR WILL BE FILED WITH THE SEC CAREFULLY AND IN THEIR ENTIRETY AS THEY BECOME AVAILABLE BECAUSE THEY CONTAIN OR WILL CONTAIN IMPORTANT INFORMATION. Investors and security holders can obtain free copies of the joint proxy statement/prospectus, the registration statement and other documents filed with the SEC by Mallinckrodt and Questcor through the website maintained by the SEC at http://www.sec.gov.

Participants in the Merger Solicitation

Mallinckrodt, Questcor, 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 Mallinckrodt and Questcor shareholders in connection with the proposed merger and a description of their direct and indirect interests, by security holdings or otherwise, is set forth in the joint proxy statement/prospectus filed by Mallinckrodt with the SEC. Information about the directors and executive officers of Mallinckrodt is set forth in its proxy statement for its 2014 annual general meeting of shareholders, which was filed with the SEC on January 24, 2014. Information about the directors and executive officers of Questcor is set forth in its amendment to Annual Report on Form 10-K/A, which was filed with the SEC on April 30, 2014.

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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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