New Study: AVONEX(R), The Most Prescribed Multiple Sclerosis Treatment Worldwide, Demonstrates Superior Adherence

Biogen Idec (NASDAQ: BIIB) announced findings today from the
Global Adherence Project (GAP) that demonstrated patients taking
AVONEX(R) (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(R) (Interferon beta-1a) sc
22mcg (22%, p=0.006), Rebif(R) 44 mcg (27%), Betaferon/Betaseron(R)
(Interferon beta-1b) (30%) and Copaxone(R) (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-Universitat, Dusseldorf, 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.