The general prescribing indication for Tysabri is for people with MS who have had an inadequate response to other disease modifying therapies or are unable to tolerate them. The approval is based on positive results from two clinical trials which showed Tysabri significantly reduced the risk of sustained progression of disability, the rate of MS relapses and reductions in MS brain lesions as shown on MRI scans in people with relapsing-remitting MS. The Health Canada decision follows the reintroduction of Tysabri in the United States in early June 2006 following an extensive safety review of the product and the approval of Tysabri in the European Union in late June. It was voluntarily withdrawn from the US market in February 2005 following the development of serious adverse side effects in three people who received Tysabri combined with Avonex® (interferon beta-1a).

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