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): June 29, 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 June 29, 2012, Catalyst Health Solutions, Inc. (“Catalyst”) entered into an Amendment to Agreement and Plan of Merger (the “Amendment”) to amend that certain Agreement and Plan of Merger, dated as of April 17, 2012 (the “Merger Agreement”), by and among Catalyst, SXC Health Solutions Corp., a corporation organized under the laws of the Yukon Territory, Canada (“SXC”), SXC Health Solutions, Inc. (“US Corp.”), Catamaran I Corp. (“Merger Sub”) and Catamaran II LLC. The Merger Agreement provides, among other things, that, upon the terms and subject to the conditions set forth therein, Merger Sub will merge with and into Catalyst, with Catalyst surviving as a wholly-owned subsidiary of US Corp. (the “Surviving Corporation”). Pursuant to the Amendment, a ministerial change has been made to the Merger Agreement and form of amended and restated certificate of incorporation of the Surviving Corporation attached as Exhibit A thereto to reduce the par value of the common stock of the Surviving Corporation. The Amendment does not affect the amount of consideration that holders of Catalyst stock would be entitled to receive for their shares upon completion of the merger. A copy of the Amendment is attached as Exhibit 2.1 and is incorporated herein by reference.

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Catalyst Forward Looking Statements

Certain statements may contain certain forward-looking statements including, without limitation, statements concerning Catalyst’s operations, economic performance and financial condition. The words “believe,” “expect,” “anticipate,” “will,” “could,” “would,” “should,” “may,” “plan,” “estimate,” “intend,” “predict,” “potential,” “continue,” and the negatives of these words and other similar expressions generally identify forward-looking statements. In addition to Catalyst’s expectations or estimates of a combined company’s future performance or matters relating to the proposed transaction with SXC, these forward-looking statements may include statements addressing Catalyst’s operations and Catalyst’s financial performance. Readers are cautioned not to place undue reliance on these forward-looking statements, which, among other things, speak only as of their dates. These forward-looking statements are based largely on Catalyst’s current expectations and are based on a number of risks and uncertainties, including, without limitation, (i) general adverse economic conditions, (ii) changes in governmental laws and regulations, (iii) Catalyst’s ability to compete effectively in the pharmacy benefit management industry, (iv) Catalyst’s relationships with key clients, pharmacy network affiliations and various pharmaceutical manufacturers and rebate intermediaries, (v) changes in industry pricing benchmarks, (vi) uncertainties relating to the transition and integration of completed and future acquisitions and/or expansion opportunities, (vii) Catalyst’s current level of indebtedness and any future indebtedness Catalyst may incur; (viii) disruption in Catalyst’s operations, (ix) unanticipated changes in Catalyst’s ability to execute its growth strategy, (x) generic utilization levels, (xi) insufficient insurance coverage to cover costs associated with litigation, (xii) Catalyst’s ability to accurately estimate how much future revenue Catalyst will generate, as well as the level of implementation and transaction costs that Catalyst will incur, under newly commenced PBM agreements and other risks and uncertainties discussed in Catalyst’s filings with the SEC, including Catalyst’s Annual Report on Form 10-K and quarterly reports on Form 10-Q. Actual results could differ materially from results referred to in the forward-looking statements. In light of these risks and uncertainties, there can be no assurances that the results referred to in the forward-looking statements contained herein will, in fact, occur. Catalyst undertakes no obligation to revise any forward-looking statements in order to reflect events or circumstances that may arise after the date hereof. Readers are urged to carefully review and consider the various disclosures made in Catalyst’s other filings with the SEC that attempt to advise interested parties of the risks and factors that may affect Catalyst’s business.

Transaction Forward-Looking Statements

In addition, numerous factors could cause actual results with respect to the proposed transaction with SXC to differ materially from those in the forward-looking statements, including without limitation, the possibility that the expected efficiencies and cost savings from the proposed transaction will not be realized, or will not be realized within the expected time period; the risk

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that the SXC and Catalyst businesses will not be integrated successfully; the ability to obtain governmental approvals of the proposed transaction on the proposed terms and schedule contemplated by the parties; the failure of shareholders of SXC or Catalyst to approve the proposed transaction; disruption from the proposed transaction making it more difficult to maintain business and operational relationships; the risk of customer attrition; the possibility that the proposed transaction does not close, including, but not limited to, due to the failure to satisfy the closing conditions; and the ability to obtain the financing contemplated to fund a portion of the consideration to be paid in the proposed transaction and the terms of such financing.

Important Additional Information

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. This communication is being made in respect of the proposed transaction involving Catalyst and SXC. The proposed transaction will be submitted to the shareholders of Catalyst and the shareholders of SXC for their consideration. On June 1, 2012, the SEC declared effective the joint proxy statement/prospectus included in the registration statement on Form S-4 filed by SXC. On or about June 4, 2012, Catalyst and SXC commenced mailing of the definitive joint proxy statement/prospectus regarding the proposed transaction. Each of the companies may be filing with the SEC other documents regarding the proposed transaction. 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 FILED WITH THE SEC CAREFULLY AND IN THEIR ENTIRETY BECAUSE THEY CONTAIN IMPORTANT INFORMATION ABOUT THE PROPOSED TRANSACTION. Investors and security holders may obtain free copies of the joint proxy statement/prospectus and other documents containing important information about Catalyst and SXC, 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. In addition, investors and security holders of Catalyst may obtain free copies of the joint proxy statement/prospectus filed with the SEC by directing a written request to:

MacKenzie Partners, Inc.

105 Madison Avenue

New York, New York 10016

Stockholders May Call Toll-Free: (800) 322-2885

Banks & Brokers May Call Collect: (212) 929-5500

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

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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 Catalyst is set forth in its proxy statement for its 2012 annual meeting of shareholders, which was filed with the SEC on April 26, 2012. 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, are contained in the joint proxy statement/prospectus and other relevant materials filed with the SEC.

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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: June 29, 2012

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EXHIBIT INDEX

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