A sixth patient taking Elan and Biogen Idec's multiple sclerosis drug Tysabri has developed a potentially deadly brain infection since the drug was reintroduced onto the market in July 2006.
US-based Biogen announced the news late on Friday on its website, saying the case was confirmed on April 15.
Biogen has said it does not plan to announce each new case of progressive multifocal leukoencephalopathy, or PML, except as a weekly update on its website.
But the confirmation occurred a day before Biogen's first-quarter earnings conference call, and some analysts questioned why the company did not mention it.
Biogen said last week that its first quarter profits rose by 50% on demand for the MS drug. Net income climbed to $243.9m while in-market sales of Tysabri came to $227m.
Sales of Tysabri have been curtailed by concerns over PML. The drug was temporarily withdrawn from the market in 2005 before being returned with stricter safety warnings.
Elan and Biogen sell Tysabri in partnership.
