National MS Society | Tysabri (Natalizumab)Warning
Tysabri increases the risk of progressive multifocal leukoencephalopathy (PML), a viral infection of the brain that usually leads to death or severe disability. Although the cases of PML that occurred in the clinical trials occurred only in patients who were also taking another immunodulating or immunosuppressing medication, there were too few cases to rule out the possibility that PML can occur in patients taking Tysabri alone.
http://www.nationalmssociety.org/Meds-Natalizumab.asp