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): May 14, 2014

ZALICUS INC.

(Exact name of Registrant as specified in its charter)

Delaware

(State or other jurisdiction of incorporation)

245 First Street

Third Floor

Cambridge, MA 02142

(Address of principal executive offices and zip code)

Registrant’s telephone number, including area code: (617) 301-7000

None

(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:

On May 14, 2014, Zalicus Inc. (“Zalicus”) entered into an Asset Purchase Agreement (the “Purchase Agreement”) with Horizon Discovery Limited, an English private limited company, and Horizon Discovery Inc., a privately held Delaware corporation (together, the “Horizon Discovery Group”), pursuant to which the Horizon Discovery Group will purchase Zalicus’ combination High Throughput Screening platform (the “cHTS Business”) and certain assets and liabilities related to the cHTS Business (the “Purchase”).

Under the terms of the Asset Purchase Agreement, Zalicus will receive $8 million from the Horizon Discovery Group at the closing of the transaction, subject to specified adjustments for net working capital. The Purchase Agreement contains customary representations, warranties and covenants by each party. Consummation of the Purchase contemplated by the Purchase Agreement is subject to the satisfaction of customary closing conditions for transactions of this type as set forth therein. The transaction is expected to close in June 2014.

The foregoing description of the Purchase and the Purchase Agreement does not purport to be complete and is qualified in its entirety by reference to the Purchase Agreement, which is filed as Exhibit 2.1 to this Current Report on Form 8-K and is incorporated herein by reference.

IMPORTANT ADDITIONAL INFORMATION AND WHERE TO FIND IT

This communication is being made in respect of the proposed merger between Zalicus and Epirus and related matters involving Zalicus and Epirus. This Form 8-K does not constitute an offer to sell or the solicitation of an offer to buy any securities or the solicitation of any vote or approval. In connection with the proposed transaction between Zalicus and Epirus, Zalicus filed a Registration Statement on Form S-4 with the Securities and Exchange Commission (the “SEC”) on May 8, 2014 (the “Registration Statement”), containing a joint proxy statement/prospectus. These materials are not yet final and will be further amended. The Registration Statement contains important information about Zalicus, Epirus, the transaction between Zalicus and Epirus and related matters. Zalicus and Epirus will mail or otherwise deliver the definitive joint proxy statement/prospectus to their respective stockholders when it becomes available. Investors and security holders of Zalicus and Epirus are urged to read carefully the joint proxy statement/prospectus relating to the merger (including any amendments or supplements thereto) in its entirety, because it contains important information about the proposed transaction.

Investors and security holders of Zalicus will be able to obtain free copies of the definitive joint proxy statement/prospectus for the proposed merger (when it is available) and other documents filed with the SEC by Zalicus through the website maintained by the SEC at www.sec.gov. In addition, investors and security holders of Zalicus will be able to obtain free copies of the definitive joint proxy statement/prospectus for the proposed merger (when it is available) by contacting Zalicus, Attn: Justin Renz, jrenz@zalicus.com. Investors and security holders of Epirus will be able to obtain free copies of the definitive joint proxy statement/prospectus for the merger by contacting Epirus, Attn: Edward Scott, escott@epirusbiopharma.com.

Zalicus and Epirus, and their respective directors and certain of their executive officers, may be deemed to be participants in the solicitation of proxies in respect of the transactions contemplated by the agreement between Zalicus and Epirus. Information regarding Zalicus’ directors and executive officers is contained in Zalicus’ Annual Report on Form 10-K for the fiscal year ended December 31, 2013, which was filed with the SEC on March 14, 2014 (as amended by Form 10-K/A which was filed with the SEC on April 30, 2014), the Registration Statement, and will also be available in the definitive joint proxy statement/prospectus that will be filed by Zalicus with the SEC in connection with the proposed transaction. Information regarding Zalicus’ directors and officers and a more complete description of the interests of Epirus’ directors and officers in the proposed transaction is available in the Registration Statement. INVESTORS AND SECURITY HOLDERS ARE URGED TO READ THE PROXY STATEMENT/ PROSPECTUS AND THE OTHER RELEVANT MATERIALS WHEN THEY BECOME AVAILABLE BEFORE MAKING ANY VOTING OR INVESTMENT DECISION WITH RESPECT TO THE PROPOSED TRANSACTIONS.

Cautionary Statement Regarding Forward-Looking Statements

Any statements made herein relating to future financial or business performance, conditions, plans, prospects, trends, or strategies and other financial and business matters, including without limitation, the ability of

Zalicus to close the transaction with the Horizon Discovery Group, the potential closing date of the transaction with Horizon Discovery Group, the amount of Zalicus’ net cash at closing for the proposed transaction with Epirus and the percentage of stock of the combined company to be received by Zalicus’ stockholders at closing of the proposed transaction with Epirus, are forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995. In addition, when or if used in this press release, the words “may,” “could,” “should,” “anticipate,” “believe,” “estimate,” “expect,” “intend,” “plan,” “predict” and similar expressions and their variants, as they relate to Zalicus or its management, before or after the aforementioned merger, may identify forward-looking statements. Zalicus caution that these forward-looking statements are subject to numerous assumptions, risks, and uncertainties, which change over time. Further information on the factors and risks that could affect Zalicus’ business, financial conditions and results of operations are contained in Zalicus’ filings with the U.S. Securities and Exchange Commission, which are available at www.sec.gov. The forward-looking statements represent the estimates of Zalicus as of the date hereof only, and Zalicus specifically disclaims any duty or obligation to update forward-looking statements.

Other risks and uncertainties are more fully described in Zalicus’ Annual Report on Form 10-K for the year ended December 31, 2013 filed with the SEC, and in other filings that Zalicus makes and will make with the SEC in connection with the proposed transactions, including the proxy statement described above under “Important Additional Information will be Filed with the SEC.” Existing and prospective investors are cautioned not to place undue reliance on these forward-looking statements, which speak only as of the date hereof. The statements made herein speak only as of the date stated herein, and subsequent events and developments may cause our expectations and beliefs to change. While we may elect to update these forward-looking statements publicly at some point in the future, we specifically disclaim any obligation to do so, whether as a result of new information, future events or otherwise, except as required by law. These forward-looking statements should not be relied upon as representing our views as of any date after the date stated herein.

Item 8.01. Other Events.

On May 15, 2014, Zalicus issued a press release announcing, among other things, the execution of the Purchase Agreement. A copy of the press release is filed as Exhibit 99.1 to this Current Report on Form 8-K and is incorporated herein by reference.

See Exhibit Index attached hereto.

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.

Dated: May 15, 2014

Index to Exhibits