Exhibit 99.1

Horizon Pharma Announces Second Quarter 2013 Financial Results and Provides Business Update

DUEXIS^® Gross Sales Increase 136% Quarter over Quarter; Sales Acceleration Extends Cash Guidance

Conference Call and Webcast Today, August 9^th at 8:00 a.m. ET

DEERFIELD, IL – August 9, 2013 – Horizon Pharma, Inc. (NASDAQ: HZNP) today provided an update on the Company’s business and announced financial results for the second quarter and six months ended June 30, 2013.

Financial Results

DUEXIS^® Highlights

RAYOS^® / LODOTRA^® Highlights

Updated Cash Guidance

“We had strong results in the second quarter, with DUEXIS revenue doubling from the first quarter, driven by continued momentum in prescriptions and a significant increase in the value per DUEXIS prescription,” said Timothy P. Walbert,

chairman, president and chief executive officer, Horizon Pharma. “DUEXIS revenue acceleration, along with our continued progress in the launch of RAYOS, gives us confidence that we can extend our cash runway into at least the third quarter of 2014 and potentially beyond based on our current expectations of continued revenue growth.”

Second Quarter 2013 Financial Results

For the second quarter ended June 30, 2013, gross and net sales were $17.6 million and $12.3 million, respectively, compared to gross and net sales of $4.6 million and $3.8 million, respectively, for the second quarter of 2012. DUEXIS gross sales were $15.8 million and net sales were $10.5 million after deducting trade discounts and allowances of $3.7 million and co-pay assistance costs of $1.6 million, compared to gross and net DUEXIS sales of $2.1 million and $1.6 million, respectively, during the second quarter of 2012. The increase in DUEXIS sales was primarily the result of the Company’s expanded sales force compared to the second quarter of 2012, as well as an increase in the product price. DUEXIS represented 89% of Horizon’s gross sales and 86% of net sales during the quarter ended June 30, 2013.

RAYOS gross sales and net sales for the second quarter of 2013 were approximately $0.7 million and $0.6 million, respectively. LODOTRA gross and net sales in the second quarter of 2013 were $1.2 million each, compared to gross and net sales of $2.5 million and $2.3 million, respectively, in the second quarter of 2012. The $1.3 million decrease in LODOTRA gross sales during the second quarter of 2013 was primarily attributable to lower product shipments to the Company’s European distribution partner, Mundipharma, compared to the prior year period. LODOTRA sales to Mundipharma occur at the time the Company ships product to Mundipharma based on its estimated requirements. Accordingly, the Company’s LODOTRA sales are not linear or tied to Mundipharma sales to the market and can therefore fluctuate from quarter to quarter or from year to year.

Net loss for the quarter ended June 30, 2013, was $18.4 million, or $0.29 per share based on 62,872,173 weighted average shares outstanding, compared to a net loss of $22.8 million, or $0.68 per share based on 33,715,703 weighted average shares outstanding, for the quarter ended June 30, 2012.

Non-GAAP net loss for the quarter ended June 30, 2013, was $15.0 million, or $0.24 per share, compared to a non-GAAP net loss of $20.6 million, or $0.62 per share, for the second quarter of 2012. Horizon provides non-GAAP financial measures, which it believes can enhance an overall understanding of its financial performance when considered together with GAAP figures. Refer to the section of this press release below entitled “Note Regarding Use of Non-GAAP Financial Measures” for a full discussion on this subject.

The Company had cash and cash equivalents of $69.3 million at June 30, 2013. As of June 30, 2013, Cowen and Company, LLC, the Company’s agent, had sold 1,266,161 shares of the Company’s common stock under its at-the-market (ATM) facility with gross proceeds to the Company of $3.2 million. As of August 6, 2013, the Company had received gross proceeds of $6.2 million under the ATM and had $21.2 million remaining available under the ATM which can potentially supplement or extend the Company’s cash runway.

Research and development expenses decreased $1.4 million, from $4.2 million during the three months ended June 30, 2012, to $2.8 million during the three months ended June 30, 2013. The decrease in research and development expenses during the second quarter of 2013 was primarily associated with the classification of $1.2 million in medical affairs expenses to sales and marketing expenses and a $0.3 million reduction in regulatory submission fees, which was partially offset by a $0.1 million increase in equipment expense. During the first quarter of 2013, in connection with the full commercial launch of RAYOS, the Company began to classify its medical affairs expenses, which consist of expenses related to scientific publications, health outcomes, biostatistics, medical education and information, and medical communications as sales and marketing expenses. Prior to the full commercial launch of RAYOS in late January 2013, these medical affairs expenses were classified as part of research and development expenses.

Sales and marketing expenses increased $6.0 million, from $10.5 during the three months ended June 30, 2012, to $16.5 during the three months ended June 30, 2013. The increase was primarily attributable to $4.5 million in salaries and benefits expenses due to the increase in staffing of the Company’s field sales force, the new classification of $1.2 million of medical affairs expenses to sales and marketing expenses and a $0.2 million increase in marketing and commercialization expenses.

General and administrative expenses increased $0.6 million, from $4.6 million during the three months ended June 30, 2012, to $5.2 million during the three months ended June 30, 2013. The increase was mainly attributable to a $0.7 million

increase in salaries and benefits expense primarily associated with new employees and $0.2 million in higher facilities costs, which was partially offset by a reduction in consulting expenses of $0.2 million.

Interest expense, net increased $0.2 million, from $3.2 million during the three months ended June 30, 2012, to $3.4 million during the three months ended June 30, 2013. The increase in interest expense was primarily attributable to higher debt discount expenses associated with an amendment to the Company’s senior secured loan in September 2012.

During the three months ended June 30, 2013, the Company reported a foreign exchange gain of $0.5 million compared to a foreign exchange loss of $1.4 million during the three months ended June 30, 2012. The foreign exchange gain in the second quarter of 2013 was primarily due to a reduction in U.S. dollar denominated transactions for the Company’s Swiss subsidiary, Horizon Pharma AG, whose functional currency is the Euro, in addition to a strengthening of the Euro against the U.S. dollar during the three months ended June 30, 2013.

Income tax benefit increased $0.2 million, from $0.2 million during the three months ended June 30, 2012, to $0.4 million during the three months ended June 30, 2013. The increase in income tax benefit during the second quarter of 2013 was primarily due to a reduction in the Company’s deferred tax liability resulting from higher intangible amortization expense in the current period. The increase in amortization expense was associated with the FDA approval of RAYOS in July 2012, which required the Company to begin amortization and to reclassify an indefinite-lived intangible asset to a definite-lived intangible asset on the balance sheet.

Year-to-Date Financial Results

For the six months ended June 30, 2013, gross and net sales were $28.3 million and $21.4 million, respectively, compared to gross and net sales of $7.5 million and $6.4 million, respectively, for the six months ended June 30, 2012. DUEXIS gross sales were $22.5 million and net sales were $15.9 million after deducting trade discounts and allowances of $4.2 million and co-pay assistance costs of $2.4 million, compared to gross and net DUEXIS sales of $3.2 million and $2.5 million, respectively, during the six months ended June 30, 2012. The increase in DUEXIS sales was primarily the result of the Company’s expanded sales force compared to the first six months of 2012, as well as product price increases. DUEXIS represented 79% of gross sales and 74% of net sales during the six months ended June 30, 2013.

RAYOS gross sales and net sales for the six months ended June 30, 2013 were approximately $1.1 million and $0.9 million, respectively, with no RAYOS sales during the corresponding period in 2012 as the product was launched in late 2012. LODOTRA gross and net sales for the six months ended June 30, 2013 were $4.7 million and $4.6 million, respectively, compared to gross and net sales of $4.3 million and $3.9 million, respectively, during the six months ended June 30, 2012.

Net loss for the six months ended June 30, 2013 was $40.6 million, or $0.65 per share based on 62,339,285 weighted average shares outstanding, compared to a net loss of $46.5 million, or $1.61 per share based on 28,909,080 weighted average shares outstanding, for the six months ended June 30, 2012.

Non-GAAP net loss for the six months ended June 30, 2013 was $33.7 million, or $0.54 per share, compared to a non-GAAP net loss of $41.2 million, or $1.43 per share, for the six months ended June 30, 2012.

Research and development expenses decreased $3.3 million, from $8.3 million during the six months ended June 30, 2012, to $5.0 million during the six months ended June 30, 2013. The decrease in research and development expenses during the six months ended June 30, 2013 was primarily associated with the classification of $2.6 million in medical affairs expenses to sales and marketing expenses, a $0.5 million reduction in expenses associated with DUEXIS clinical studies and a $0.5 million decrease in consulting expenses, which was partially offset by a $0.2 million increase in equipment expenses and $0.1 million in additional salaries and benefits costs.

Sales and marketing expenses increased $11.3 million, from $21.5 million during the six months ended June 30, 2012, to $32.8 million during the six months ended June 30, 2013. The increase was primarily attributable to $8.8 million in salaries and benefits expenses due to the expansion of the Company’s field sales force and the classification of $2.6 million of medical affairs expenses to sales and marketing expenses during the six months ended June 30, 2013.

General and administrative expenses increased $0.3 million, from $9.8 million during the six months ended June 30, 2012, to $10.1 million during the six months ended June 30, 2013. The increase was mainly due to $0.5 million in higher salaries and benefits expenses primarily associated with new employees, a $0.4 million increase in facilities costs and $0.2 million

in higher legal fees associated with intellectual property related matters, which was primarily offset by a $0.7 million reduction in professional fees compared to the prior year.

Interest expense, net decreased $0.7 million, from $7.7 million during the six months ended June 30, 2012, to $7.0 million during the six months ended June 30, 2013. The decrease in interest expense was primarily attributable to the absence of debt extinguishment costs in 2013, partially offset by higher debt discount expenses. During the six months ended June 30, 2012, the Company incurred approximately $2.5 million in pre-payment and end of loan payments associated with the extinguishment of its prior debt facilities.

During the six months ended June 30, 2013 and 2012, the Company reported a foreign exchange loss of $0.5 million and $0.9 million, respectively. The decrease in the foreign exchange loss during the six months ended June 30, 2013 was primarily due to a reduction in U.S. dollar denominated transactions for the Company’s Swiss subsidiary, Horizon Pharma AG, whose functional currency is the Euro, in addition to a strengthening of the Euro against the U.S. dollar in 2013.

Income tax benefit increased $0.9 million, from $0.3 million during the six months ended June 30, 2012, to $1.2 million during the six months ended June 30, 2013. The increase in income tax benefit during the six months ended June 30, 2013 was primarily due to a reduction in the Company’s deferred tax valuation allowance resulting from a determination that a greater portion of deferred tax assets associated with deferred revenues from milestone payments would be realized in future years. Accordingly, the Company recorded an additional income tax benefit of $0.8 million during the six months ended June 30, 2013 to properly account for the realization of its deferred tax asset positions.

Note Regarding Use of Non-GAAP Financial Measures

Horizon provides non-GAAP net income (loss) and net income (loss) per share financial measures that include adjustments to GAAP figures. These adjustments to GAAP exclude non-cash items such as stock compensation and depreciation and amortization, non-cash interest expense, and other non-cash charges. Horizon believes that these non-GAAP financial measures, when considered together with the GAAP figures, can enhance an overall understanding of Horizon’s financial performance. The non-GAAP financial measures are included with the intent of providing investors with a more complete understanding of operational results and trends. In addition, these non-GAAP financial measures are among the indicators Horizon’s management uses for planning and forecasting purposes and measuring the Company’s performance. These non-GAAP financial measures should be considered in addition to, and not as a substitute for, or superior to, financial measures calculated in accordance with GAAP. The non-GAAP financial measures used by the Company may be calculated differently from, and therefore may not be comparable to, non-GAAP financial measures used by other companies. Please refer to the financial statements portion of this press release for a reconciliation of these non-GAAP financial measures to the comparable GAAP financial measures.

Conference Call

At 8:00 a.m. Eastern Time today, Horizon’s management will host a live conference call and webcast to review the Company’s financial and operating results and provide a general business update.

The live webcast and a replay may be accessed by visiting Horizon’s website at http://ir.horizon-pharma.com. Please connect to the Company’s website at least 15 minutes prior to the live webcast to ensure adequate time for any software download that may be needed to access the webcast. Alternatively, please call 1-888-338-8373 (U.S.) or 973-872-3000 (international) to listen to the conference call. The conference ID number for the live call is 11632898. Telephone replay will be available approximately two hours after the call. To access the replay, please call 1-855-859-2056 (U.S.) or 404-537-3406 (international). The conference ID number for the replay is 11632898.

About Horizon Pharma

Horizon Pharma, Inc. is a specialty pharmaceutical company that has developed and is commercializing DUEXIS and RAYOS/LODOTRA, both of which target unmet therapeutic needs in arthritis, pain and inflammatory diseases. The Company’s strategy is to develop, acquire, in-license and/or co-promote additional innovative medicines where it can execute a targeted commercial strategy in specific therapeutic areas while taking advantage of its commercial strengths and the infrastructure the Company has put in place. For more information, please visit www.horizonpharma.com.

Forward-Looking Statements

This press release contains forward-looking statements, including statements regarding the on-going commercial launches of DUEXIS and RAYOS, trends with respect to DUEXIS revenue acceleration and DUEXIS and RAYOS prescription growth, the momentum for DUEXIS and RAYOS, and estimates of Horizon’s cash runway and future impact of potential revenue growth. These forward-looking statements are based on management’s expectations and assumptions as of the date of this press release and actual results may differ materially from those in these forward-looking statements as a result of various factors. These factors include, but are not limited to risks regarding Horizon’s ability to commercialize products successfully, whether commercial data regarding DUEXIS and RAYOS in the United States for any historic periods are indicative of future results, the impact of pricing decisions on product revenues, Horizon’s ability to successfully manage contract sales and marketing personnel, Horizon’s ability to comply with post-approval regulatory requirements, the need to potentially obtain additional financing to successfully commercialize or further develop DUEXIS and RAYOS/LODOTRA and potential that Horizon may use cash resources in ways not currently anticipated or planned. For a further description of these and other risks facing the Company, please see the risk factors described in the Company’s filings with the United States Securities and Exchange Commission, including those factors discussed under the caption “Risk Factors” in those filings. Forward-looking statements speak only as of the date of this press release and the Company undertakes no obligation to update or revise these statements, except as may be required by law.

CONDENSED CONSOLIDATED BALANCE SHEETS

(in thousands, except share and per share data)

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS

(in thousands, except share and per share data)

RECONCILIATION OF GAAP NET LOSS TO NON-GAAP NET LOSS

(in thousands, except share and per share amounts)

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS

(in thousands)

Contacts

Robert J. De Vaere

Executive Vice President and Chief Financial Officer

investor-relations@horizonpharma.com

Investors

Kathy Galante

Burns McClellan, Inc.

212-213-0006

kgalante@burnsmc.com