New Study: AVONEX®, The Most Prescribed Multiple Sclerosis Treatment Worldwide, Demonstrates Superior Adherence

Cambridge, MA - September 28, 2006 - Biogen Idec (NASDAQ: BIIB) announced findings today from the Global Adherence Project (GAP) that demonstrated patients taking AVONEX® (interferon beta-1a), the most prescribed multiple sclerosis (MS) treatment worldwide, adhered to treatment more than patients taking other approved disease-modifying therapies (DMTs) included in the study. The multiple sclerosis study results were announced at the 22nd Congress of the European Committee for Treatment and Research in Multiple Sclerosis (ECTRIMS) in Madrid, Spain.

GAP is the largest study of its kind to evaluate patient adherence to long-term treatments for multiple sclerosis in a real-world setting with 176 sites across 22 countries participating. The cross-sectional observational study enrolled 2,566 patients with relapsing-remitting MS taking one of the DMTs. Neurologists completed a practice-related survey and patients completed a patient survey on their treatment adherence practices, plus the MS International Quality of Life Questionnaire and the MS Neuropsychological Screening Questionnaire. Non-adherence was defined as missing at least one injection or changing dose within four weeks prior to the survey. The World Health Organization (WHO) has defined treatment adherence as both compliance (taking a medication in the dose and according to the schedule prescribed) and persistency (maintenance of the drug regimen over the long-term), and suggests that non-adherence is one of the most important factors contributing to decreased multiple sclerosis treatment effectiveness.

The GAP study found that patients taking AVONEX had statistically significant higher adherence rates than the other DMTs. Results showed that 85% of patients taking AVONEX were adherent to therapy, compared to patients who were non-adherent on Rebif® (Interferon beta-1a) sc 22mcg (22%, p=0.006), Rebif® 44 mcg (27%), Betaferon/Betaseron® (Interferon beta-1b) (30%) and Copaxone® (glatiramer acetate) (34%), (all p>0.0001). In addition to the high adherence rate seen in the GAP study, AVONEX is the only available once-weekly MS treatment that has been proven to delay the risk of developing clinically definite MS for up to five years in patients who begin treatment immediately after their initial MS attack.

“Adherence is a critical component in chronic disease treatment, directly impacting long-term efficacy. The GAP study suggests that AVONEX leads to superior adherence which may contribute to long-term effectiveness,” said Bernd Kieseier, MD, Professor of Neurology, Heinrich-Heine-Universität, Düsseldorf, Germany, and a lead investigator in the project. “The study also demonstrated that therapeutic choice can directly affect a patient’s adherence to multiple sclerosis treatment, with factors such as frequency of drug administration, how the therapy is administered, side effects, and how effective the patient perceives their treatment also playing a role.”

The study showed that adherent patients reported better quality of life, less cognitive impairment and fewer problems with injection site reactions that non-adherent patients. Findings also demonstrated that the most common reason for non-adherence to multiple sclerosis treatment was forgetting to administer the injection (50%). Other factors affecting adherence included duration of current therapeutic treatment and duration of the disease.

“The GAP study aimed to characterize those factors that are associated with adherent versus less adherent MS disease-modifying agent injection-taking behavior. These results have substantial implications in terms of our ability to anticipate difficulties that patients may have with specific therapeutic regimens, and for disease-modifying injection therapies in general,” said Eliot Frohman, MD, PhD, Professor of Neurology and Ophthalmology, and Director, Multiple Sclerosis Program and Vertigo and Eye Movement Clinics at University of Texas Southwestern Medical Center.
About AVONEX

AVONEX is the most prescribed treatment for relapsing forms of multiple sclerosis 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 multiple sclerosis treatment approved for patients who have their first clinical MS attack and have a brain MRI scan that suggests 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 in caution with 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 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 companies’ current beliefs and expectation. No forward-looking statement can be guaranteed. Drug development involves a high degree of risk. Factors which could cause actual results to differ materially from the companies’ 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.

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