Exhibit 99.1

FOR IMMEDIATE RELEASE

Idenix Pharmaceuticals Contacts:

Kelly Barry (617) 995-9033 (media)

Teri Dahlman (617) 995-9807 (investors)

IDENIX PHARMACEUTICALS REPORTS FIRST QUARTER 2012 FINANCIAL

RESULTS AND HCV DEVELOPMENT PROGRAM PROGRESS

CAMBRIDGE, Mass., May 2, 2012 — Idenix Pharmaceuticals, Inc. (NASDAQ: IDIX), a biopharmaceutical company engaged in the discovery and development of drugs for the treatment of human viral diseases, today reported unaudited financial results for the first quarter ended March 31, 2012 and progress of its hepatitis C virus (HCV) development programs.

Operational Highlights

IDX184 Program, an HCV nucleotide polymerase inhibitor

IDX719 Program, an HCV NS5A inhibitor

Nucleotide Discovery Program

“The first quarter has been a productive one for Idenix as our three HCV development programs continued to move forward,” said Ron Renaud, Idenix’s President and Chief Executive Officer. “We are on track to achieve our near-term goals of initiating our first all-oral DAA combination trial for IDX184 and a protease inhibitor, generating IDX719 proof-of-concept data and moving IDX19368 into the clinic. As we continue to advance our programs, we continue to build on the promising profiles of our HCV candidates as potential components of future HCV combination treatments.”

First Quarter Ended 2012 Financial Results

For the first quarter ended March 31, 2012, Idenix reported total revenues of $35.6 million, compared to total revenues of $4.0 million in the first quarter of 2011. The Company reported a net income of $11.4 million, or $0.11 per basic share and $0.10 per diluted share, for the first quarter ended March 31, 2012, compared to a net loss of $8.2 million, or $0.11 per basic and diluted share for the first quarter ended March 31, 2011. In the first quarter ended March 31 2012, Idenix reported net income due to the recognition of $36.1 million of deferred revenue related to the termination of the license agreement with ViiV Healthcare Company on March 15, 2012. However, the Company expects to report a net loss for the year ending December 31, 2012 and for the next several years as it continues to expand its drug discovery and development efforts.

2012 Financial Guidance

At March 31, 2012, Idenix’s cash and cash equivalents totaled $105.3 million. The Company expects that its current cash, cash equivalents and the anticipated royalty payments associated with product sales of Tyzeka^®/Sebivo^® (telbivudine) will be sufficient to sustain its operations through at least December 31, 2012. This guidance assumes no milestone payments, license fees, reimbursement for development programs and no financing activities.

ABOUT IDENIX

Idenix Pharmaceuticals, Inc., headquartered in Cambridge, Massachusetts, is a biopharmaceutical Company engaged in the discovery and development of drugs for the treatment of human viral diseases. Idenix’s current focus is on the treatment of patients with hepatitis C infection. For further information about Idenix, please refer to www.idenix.com.

FORWARD-LOOKING STATEMENTS

This press release contains “forward-looking statements” for purposes of the safe harbor provisions of The Private Securities Litigation Reform Act of 1995, including but not limited to the statements regarding the Company’s future business and financial performance. For this purpose, any statements contained herein that are not statements of historical fact may be deemed forward-looking statements. Without limiting the foregoing, the words “expect,” “plans,” “anticipates,” “intends,” “will,” and similar expressions are also intended to identify forward-looking statements, as are expressed or implied statements with respect to the Company’s potential pipeline candidates, including any expressed or implied statements regarding the efficacy and safety of IDX184, IDX719, IDX19368 or any other drug candidate; the successful development of novel combinations of direct-acting antivirals for the treatment of HCV; the likelihood and success of any future clinical trials involving our drug candidates; and expectations with respect to future milestone or royalty payments, funding of operations and future cash balances. Actual results may differ materially from those indicated by such forward-looking statements as a result of risks and uncertainties, including but not limited to the following: there can be no guarantees that the Company will advance any clinical product candidate or other component of its potential pipeline to the clinic, to the regulatory process or to commercialization; management’s expectations could be affected by unexpected regulatory actions or delays; uncertainties relating to, or unsuccessful results of, clinical trials, including additional data relating to the ongoing clinical trials evaluating its product candidates; the Company’s ability to obtain additional funding required to conduct its research, development and commercialization activities; the Company’s dependence on its collaboration with Novartis; changes in the Company’s business plan or objectives; the ability of the Company to attract and retain qualified personnel; competition in general; and the Company’s ability to obtain, maintain and enforce patent and other intellectual property protection for its product candidates and its discoveries. Such forward-looking statements involve known and unknown risks, uncertainties and other factors that may cause actual results to be materially different from any future results, performance or achievements expressed or implied by such statements. These and other risks which may impact management’s expectations are described in greater detail under the heading “Risk Factors” in the Company’s annual report on Form 10-K for the year ended December 31, 2011 as filed with the Securities and Exchange Commission (SEC) and in any subsequent periodic or current report that the Company files with the SEC.

All forward-looking statements reflect the Company’s estimates only as of the date of this release (unless another date is indicated) and should not be relied upon as reflecting the Company’s views, expectations or beliefs at any date subsequent to the date of this release. While Idenix may elect to update these forward-looking statements at some point in the future, it specifically disclaims any obligation to do so, even if the Company’s estimates change.

IDENIX PHARMACEUTICALS, INC.

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND

COMPREHENSIVE INCOME (LOSS)

(IN THOUSANDS, EXCEPT PER SHARE DATA)

(UNAUDITED)

IDENIX PHARMACEUTICALS, INC.

CONDENSED CONSOLIDATED BALANCE SHEETS

(IN THOUSANDS)

(UNAUDITED)