More than 100 cases of a potentially fatal brain infection have been reported among patients who took Biogen Idec's multiple sclerosis drug Tysabri, US health officials said.

The Food and Drug Administration also said it was updating prescribing instructions for Tysabri to warn doctors about how the risk for the brain infection, progressive multiform leukoencephalopathy (PML), changes over time.

Biogen, which makes Tysabri with Irish drugmaker Elan Corp, temporarily withdrew the medicine in 2005 after the first PML cases. The drug returned to the market with stricter warnings in 2006, although concerns about PML have crimped sales. – Reuters