After the FDA approved Tysabri, the drug's maker took it off the market due to reports of PML (progressive multifocal leukoencephalopathy), a rare but serious brain infection. When safety programs were put in place, the drug went back on the market. The risk of PML is higher if you�ve taken drugs that turn down your immune system. Doctors usually recommend Tysabri only if other MS treatments haven�t worked. Other side effects include liver damage and life-threatening infections.