About 100 patients who took one of Biogen's (BIIB) multiple sclerosis drugs suffered from a rare but serious brain infection, according to fresh data released by regulators Friday. 

The Food and Drug Administration is updating the label on Tysabri to reflect new findings that show patients who took immune-suppressing medications before starting Tysabri are at a higher risk for getting a brain infection called progressive multifocal leukoencephalopathy, or PML. The label will also be updated with information showing the risk for PML increases after more than 24 infusions are administered. 

Regulators were already aware taking the drugs in combination could cause an increased risk for PML.  

Despite the new information, "the FDA continues to believe that the benefits of taking Tysabri outweigh the potential risks," the report said. 

Tysabri is an immunomodulator, which means it prevents certain types of cells from damaging the immune system.  It was originally approved by the FDA to treat relapsing forms of MS in 2004 and was also approved to treat severely active Crohn's disease in 2008. 

Biogen revealed strong first-quarter earnings Thursday and said its experimental BG-12 oral MS treatment showed strong effectiveness, potentially poising the company to compete with other companies like Novartis (NVS) and Teva Pharmaceuticals (TEVA) who are working on similar medications.

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