Progressive multifocal leukoencephalopathy (PML) is a serious and often fatal condition caused by a viral infection in the white matter of the brain at multiple locations. It occurs almost exclusively in people with severe immune deficiency, . transplant patients on immunosuppressive medications, or AIDS patients. It is a rare side affect for people taking the MS drug natalizumab/Tysabri. The risk of developing PML is about one in 1,000 for people who taking natalizumab for more than 12 months. The risk increases after 2 years of therapy. Patients with MS should be informed about the risks of natalizumab, including PML, both before treatment and again after 2 years. They will be monitored regularly to identify PML and other potentially serious side effects as early as possible.
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