updated 2/5/2010 2:30:31 PM ET 2010-02-05T19:30:31

Federal health officials warned doctors Friday that the risk of a deadly brain inflammation linked to the multiple sclerosis treatment Tysabri increases with use of the drug.

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Tysabri has been plagued for years by rare cases of progressive multifocal leukoencephalopathy, or PML, which can cause swelling of the brain that is usually fatal. It is mostly seen in patients with immune deficiencies.

The biotech drug is marketed by Cambridge, Mass.-based Biogen Idec Inc. and Irish drugmaker Elan Corp. PLC, for multiple sclerosis and Crohn's disease.

The Food and Drug Administration said in a statement it has confirmed a total of 31 cases of PML since mid-2006. Eight of those patients have died, as of last month, according to the FDA. The agency posted updated labeling information for the drug to its Web site.

In the new labeling, FDA notes there have been no reports of PML in patients taking Tysabri for less than 12 months. In patients taking the drug for two to three years the rate of PML is one case per 1,000 patients.

The new labeling also warns of a complication of PML, known as Immune Reconstitution Inflammatory Syndrome, that can dramatically damage patients' health even after they have stopped taking Tysabri. The agency urges doctors to monitor patients for signs of inflammation.

Tysabri was approved in November 2004 and pulled from the market the next year due to PML reports. It was reintroduced in July 2006, and has been used by 66,000 people worldwide.

Shares of Biogen Idec Inc. fell 4 cents Friday to $53.06 in midday trading. Elan dipped 3 cents to $7.11.

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