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 25, 2013

Arena Pharmaceuticals, Inc.

(Exact name of registrant as specified in its charter)

6154 Nancy Ridge Drive, San Diego, California 92121

(Address of principal executive offices) (Zip Code)

858.453.7200

(Registrant’s telephone number, including area code)

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:

In this report, “Arena Pharmaceuticals,” “Arena,” “Company,” “we,” “us” and “our” refer to Arena Pharmaceuticals, Inc., and our wholly owned subsidiaries, unless the context otherwise provides. BELVIQ^® is the trade name for lorcaserin hydrochloride in the United States. Lorcaserin hydrochloride may in the future be marketed outside of the United States as BELVIQ or under a different trade name. Arena Pharmaceuticals^® and Arena^® are registered service marks of Arena Pharmaceuticals, Inc. BELVIQ is a registered trademark of Arena Pharmaceuticals GmbH in the United States and Taiwan.

On July 25, 2013, we announced that our wholly owned subsidiary, Arena Pharmaceuticals GmbH, has entered into an exclusive marketing and supply agreement with CY Biotech Company Ltd., or CYB, for BELVIQ in Taiwan. Under the agreement, we granted CYB the rights to market and distribute BELVIQ in Taiwan for weight loss or weight management in obese and overweight patients, subject to regulatory approval of BELVIQ by the Taiwan Food and Drug Administration, or TFDA.

We will manufacture BELVIQ at our facility in Switzerland, and sell finished product to CYB for a purchase price at 45% of CYB’s annual net sales. In addition, we received from CYB a net upfront payment of $2 million, and are eligible to receive purchase price adjustment payments based on CYB’s annual net sales, as well as a milestone payment upon approval of the first additional indication for BELVIQ by the TFDA. CYB is responsible for the development, regulatory approval and, ultimately, marketing and distribution of BELVIQ in Taiwan, including related costs and expenses.

Forward-Looking Statements

Certain statements in this Form 8-K are forward-looking statements that involve a number of risks and uncertainties. Such forward-looking statements include statements about the advancement, therapeutic indication and use, and regulatory review and approval of BELVIQ; marketing of lorcaserin hydrochloride outside of the United States; rights and obligations under the marketing and supply agreement with CYB; and expectations and future activities related to such agreement, including the development, regulatory approval, manufacture, sale, marketing and distribution of BELVIQ and upfront, milestone and other payments. For such statements, we claim the protection of the Private Securities Litigation Reform Act of 1995. Actual events or results may differ materially from our expectations. Factors that could cause actual results to differ materially from the forward-looking statements include, but are not limited to, the following: risks related to the implementation and continuation of the marketing and supply agreement with CYB and relying on collaborative arrangements; risks related to commercializing drugs, including regulatory, manufacturing and supply issues and the availability and use of BELVIQ; cash and revenues generated from BELVIQ, including the impact of competition; our revenues will be based in part on management’s estimates, judgment and accounting policies, and incorrect estimates or disagreement regarding our estimates or accounting policies may result in changes to our guidance or previously reported results; the timing and outcome of regulatory review is uncertain, and BELVIQ may not be approved for marketing when expected or ever in combination with another drug, for another indication or using a different formulation or in any other territory for any indication; regulatory decisions in one territory may impact other regulatory decisions and our business prospects; government and commercial reimbursement and pricing decisions; the timing and receipt of payments and fees, if any, from collaborators; the entry into or modification or termination of collaborative arrangements; unexpected or unfavorable new data; nonclinical and clinical data is voluminous and detailed, and regulatory agencies may interpret or weigh the importance of data differently and reach different conclusions than us or others, request additional information, have additional recommendations or change their guidance or requirements before or after approval; data and other information related to any of our research and development may not meet regulatory requirements or otherwise be sufficient for further research and development, regulatory review or approval or continued marketing; our ability to obtain and defend patents; the timing, success and cost of our research

and development; results of clinical trials and other studies are subject to different interpretations and may not be predictive of future results; clinical trials and other studies may not proceed at the time or in the manner expected or at all; having adequate funds; and satisfactory resolution of litigation or other disagreements with others. Additional factors that could cause actual results to differ materially from those stated or implied by our forward-looking statements are disclosed in our filings with the Securities and Exchange Commission. These forward-looking statements represent our judgment as of the time of the filing of this Form 8-K. We disclaim any intent or obligation to update these forward-looking statements, other than as may be required under applicable law.

SIGNATURE

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.

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