In January, the U.S. Food and Drug Administration expanded the use of Tysabri to treat Crohn's disease, a chronic inflammation of the digestive tract.

The drug has also been linked with serious brain infections in multiple sclerosis patients, as well as liver damage.

When the FDA granted approval to use Tysabri for Crohn's disease, it asked the manufacturers -- Biogen Idec of Cambridge, Mass., and Elan Corp. -- to send letters to health-care providers about the potential problems with liver toxicity.

"This [the Biogen Idec letter] was part of the approval of Jan. 14," said FDA spokeswoman Rita Chappelle. "The manufacturer was going to send out a notice to health-care providers to make them aware of the potential problem."

A warning about the danger of Tysabri to the liver is already on the product label.

"FDA does not require 'Dear Doctor Letters,' but can request that a sponsor send one to inform health-care practitioners about new adverse events that are labeled," Chappelle added.

According to the letter from Biogen Idec, the drug should be stopped or discontinued if patients become jaundiced or have other evidence of liver damage.

Tysabri (natalizumab), a monoclonal antibody used to treat MS, has been under a cloud of controversy for some time. The drug works by attaching itself to white blood cells called lymphocytes and preventing them from entering the brain, where they do damage that causes the disabling symptoms of MS.

But the drug has a checkered past. It first received FDA approval in November 2004, only to be pulled from the market three months later, after several patients in clinical trials developed a rare but deadly viral infection of the brain called progressive multifocal leukoencephalopathy.

In June 2006, the FDA allowed the drug back on the market but with strict conditions. According to those revised guidelines, Tysabri can only be administered by approved doctors, at infusion sites and pharmacies that register and comply with a patient-safety program called CD Touch, designed by Biogen Idec and approved by the FDA.

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