AVONEX(R), the Most Prescribed Multiple Sclerosis Therapy Worldwide, Launched in Japan.

Biogen Idec Extends its Commercial Capabilities in Asia, Offering Important Therapy to Japanese MS Community

CAMBRIDGE, Mass. -- Biogen Idec (NASDAQ: BIIB) announced today that AVONEX([R]) (Interferon beta-1a), the most prescribed multiple sclerosis (MS) therapy worldwide, is now available in Japan. AVONEX is available in more than 90 countries, and its introduction in Japan extends Biogen Idec's leadership in the treatment of MS to an important and underserved market.

In July 2006, the Japanese Ministry of Health, Labour and Welfare approved AVONEX for the prevention of MS relapse. AVONEX is the first new MS treatment available in Japan in six years. It is the second disease-modifying therapy approved to treat MS in Japan and the only one that can be administered once a week. In international clinical trials, AVONEX has been proven to slow the progression of disability and reduce the frequency of clinical relapses, as well as delay the development of clinically definite MS.1

"For more than a decade, Biogen Idec has been committed to advancing the treatment of MS, and we look forward to extending that dedication to serve patients and their families in Japan," said James C. Mullen, President and Chief Executive Officer, Biogen Idec. "The introduction of AVONEX to the Japanese MS community continues our mission of delivering important therapeutics to patients around the world."

There are approximately 12,000 people diagnosed with MS in Japan.2However, a number of patients are believed to be undiagnosed. In addition, many Japanese patients are treated with acute rather than long-term therapy, and there are a number of patients who have discontinued therapy due to perceived lack of efficacy or side effects. Initially, AVONEX will be introduced in Japan to hospitals with MS experts, and then Biogen Idec expects it will become more widely available.

"AVONEX is the leading choice for prescribing physicians and their MS patients because it offers proven efficacy and a convenient dosing regimen that patients can adhere to over the long term," said Toshio Nakata, President of Biogen Idec Japan. "MS is a debilitating disease that remains under-treated in Japan, and we believe patients can benefit greatly from the availability of a new treatment option."

About AVONEX

AVONEX is the most prescribed treatment for relapsing forms of MS worldwide, with more than 130,000 patients on therapy. It was launched in the U.S. in 1996 and later in Europe for the treatment of relapsing forms of MS to slow the progression of disability and reduce relapses. AVONEX is marketed internationally in more than 90 countries. AVONEX was the first treatment approved for patients who have their first clinical MS attack and have a brain MRI scan consistent with MS; this use was approved in Europe in 2002 and in the U.S. in 2003.

The most common side effects associated with AVONEX multiple sclerosis treatment are flu-like symptoms, including myalgia, fever, fatigue, headache, chills, nausea, vomiting, pain and asthenia.

AVONEX should be used with caution in patients with depression or other mood disorders and in patients with seizure disorders. AVONEX should not be used by pregnant women. Patients with cardiac disease should be closely monitored. Patients should also be monitored for signs of hepatic injury. Routine periodic blood chemistry and hematology tests are recommended during treatment with AVONEX. Rare cases of anaphylaxis have been reported. Please see complete prescribing information available at www.AVONEX.com.

About Multiple Sclerosis

MS is a chronic disease of the central nervous system that affects approximately 400,000 people in North America and more than one million people worldwide. It is a disease that affects more women than men, with onset typically occurring between 20 and 50 years of age. MS is caused by damage to myelin, the protective sheath surrounding nerve fibers in the central nervous system, which interferes with messages from the brain to the body. Symptoms of MS may include vision problems, loss of balance, numbness, difficulty walking and paralysis.

About Biogen Idec

Biogen Idec creates new standards of care in oncology, neurology and immunology. As a global leader in the development, manufacturing, and commercialization of novel therapies, Biogen Idec transforms scientific discoveries into advances in human healthcare. For product labeling, press releases and additional information about the company, please visit www.biogenidec.com.

Safe Harbor/Forward-Looking Statements

This press release contains forward-looking statements regarding AVONEX that are subject to a number of risks and uncertainties. These statements are based on the company's current beliefs and expectations. No forward-looking statement can be guaranteed. Drug development involves a high degree of risk. Factors that could cause actual results to differ materially from the company's current expectations include: the risk that unexpected concerns may arise from additional data or analysis or that the company may encounter other unexpected hurdles. For more detailed information on the risks and uncertainties associated with Biogen Idec's drug development and other activities, see the periodic reports of Biogen Idec Inc. filed with the Securities and Exchange Commission. Biogen Idec assumes no obligation to update any forward-looking statements, whether as a result of new information, future events or otherwise.

1 AVONEX[R] (Interferon beta-1a) prescribing information

2 Japanese Intractable Diseases Information Center, 2006. http://www.nanbyou.or.jp/what/nan_kouhu1.htm