Q: What are the possible side effects of Tysabri?

A: Tysabri was temporarily removed from the market in 2005 after several patients in clinical trials developed progressive multifocal leukoencephalopathy (PML), a rare and deadly brain infection. The Food and Drug Administration (FDA) has also warned that some patients taking Tysabri show clinical signs of severe liver damage less than a week after their first dose. This liver damage could lead to death or require the patient to get a liver transplant.