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): April 17, 2012

CATALYST HEALTH SOLUTIONS, INC.

(Exact name of registrant as specified in its charter)

800 King Farm Boulevard

Rockville, Maryland

20850

(Address of Principal Executive Offices)

(Zip Code)

Registrant’s telephone number, including area code: (301) 548-2900

N/A

(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 (see General Instruction A.2. below):

On April 18, 2012, Catalyst Health Solutions, Inc. (“Catalyst”) and SXC Health Solutions Corp. (“SXC”) issued a joint press release regarding the execution of an Agreement and Plan of Merger (the “Merger Agreement”), dated April 17, 2012, by and among, SXC, SXC Health Solutions, Inc., Catamaran I Corp., Catamaran II LLC, and Catalyst. A copy of the press release is attached as Exhibit 99.1, and is incorporated herein by reference. As detailed more fully in the joint press release, SXC and Catalyst will host a conference call on April 18, 2012, at 8:30 a.m. Eastern time to discuss the combination. In connection with the conference call, a copy of the investor slides are attached as Exhibit 99.2 and are incorporated herein by reference.

Important Additional Information

This filing 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. This filing is being made in respect of the proposed transaction involving Catalyst and SXC. The proposed transaction will be submitted to the stockholders of Catalyst and the shareholders of SXC for their consideration. In connection therewith, the parties intend to file relevant materials with the SEC, including a joint proxy statement/prospectus that will be mailed to shareholders. Such documents, however, are not currently available. BEFORE MAKING ANY VOTING OR INVESTMENT DECISIONS, INVESTORS AND SECURITY HOLDERS OF CATALYST AND/OR SXC ARE URGED TO READ THE JOINT PROXY STATEMENT/PROSPECTUS REGARDING THE PROPOSED TRANSACTION AND ANY OTHER RELEVANT DOCUMENTS THAT WILL BE FILED WITH THE SEC CAREFULLY AND IN THEIR ENTIRETY WHEN THEY BECOME AVAILABLE BECAUSE THEY WILL CONTAIN IMPORTANT INFORMATION ABOUT THE PROPOSED TRANSACTION. Investors and security holders may obtain free copies of the proxy statement/prospectus and other documents containing important information about Catalyst and SXC, once such documents are filed with the SEC, through the website maintained by the SEC at www.sec.gov. Copies of the documents filed with the SEC by SXC will be available free of charge on SXC’s website at www.sxc.com under the heading “Investor Information” or by contacting SXC’s Investor Relations Department at 630-577-3100. Copies of the documents filed with the SEC by Catalyst will be available free of charge on Catalyst’s website at www.catalysthealthsolutions.com under the heading “Investor Information” or by contacting Catalyst’s Investor Relations Department at 301-548-2900.

SXC, Catalyst and certain of their respective directors, executive officers and other members of management and employees may be deemed to be participants in the solicitation of proxies in connection with the proposed transaction. Information about the directors and executive officers of SXC is set forth in its proxy statement for its 2012 annual meeting of stockholders, which was filed with the SEC on April 2, 2012. Information about the directors and executive officers of

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Catalyst is set forth in its proxy statement for its 2011 annual meeting of shareholders, which was filed with the SEC on April 28, 2011. These documents can be obtained free of charge from the sources indicated above. Other information regarding the participants in the proxy solicitation and a description of their direct and indirect interests, by security holdings or otherwise, will be contained in the joint proxy statement/prospectus and other relevant materials to be filed with the SEC when they become available.

The following exhibits are being filed herewith:

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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.

Date: April 18, 2012

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Exhibit Index

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