In 2010, the U.S Food and Drug Administration added a new warning about the rare brain infection to Tysabri's label. The drug also is used to treat Crohn's disease.
In January, the FDA approved a test to determine the risk of brain infection in Tysabri users.
In the new study, the researchers examined various statistics to figure out which multiple sclerosis patients were at highest risk of progressive multifocal leukoencephalopathy if they take Tysabri. Blood samples of nearly 5,900 patients with multiple sclerosis were analyzed.
Just more than 1 percent of patients showed signs of exposure to the JC virus, had taken immune-suppressing drugs before Tysabri and had been on Tysabri for 25 to 48 months.
The risk for those who hadn't been exposed to the JC virus was 120 times smaller at 0.009 percent.
Patients who worry about progressive multifocal leukoencephalopathy "may think about it differently because of the low risk in that subgroup of patients," Bloomgren said.
Multiple sclerosis patients who have been exposed to the JC virus but haven't taken immune-suppressing drugs while on Tysabri may choose to continue taking the drug, said Dr. Scott Zamvil, a professor of neurology at the University of California-San Francisco.
"Patients who are on the drug don't want to come off because it's that potent of a drug," he said. "It is the most potent of the currently approved drugs."
Patients need to talk to their physicians and weigh the benefit of the medication versus the potential risk, Zamvil said.
The study appears in the May 17 issue of the New England Journal of Medicine.
For more about multiple sclerosis, try the U.S. National Library of Medicine.
SOURCES: Gary Bloomgren, M.D., vice president of drug safety, and Sandr
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