The US Food and Drug Administration is again alerting patients and doctors to a rare but serious side effect of Rituxan (rituximab), a drug approved to treat non-Hodgkin lymphoma.

According to the agency, people taking Rituxan may be at risk of developing a severe viral infection called progressive multifocal leukoencephalopathy, or PML. The infection, which can be fatal, can develop as long as 12 months after treatment has stopped.

Rituxan's label already contains a warning about PML. There have been 23 confirmed cases of PML in people with lymphoma who were treated with Rituxan, according to the FDA.

But lymphoma patients shouldn't be overly concerned by the renewed warning, says Philip Bierman, MD, a hematologist at the University of Nebraska Medical Center who treats patients with Rituxan.

"It's something that we need to be aware of," he says, "but the bottom line is, if your physician thinks you need rituximab, probably the benefits outweigh the risks of this very rare complication."

Lymphoma patients should not stop taking the drug, he adds. Those who are concerned should talk with their doctor about PML.

Other experts agree. Rituxan has helped people live longer, and patients should not abandon it over fears about a rare complication, says lymphoma expert Bruce D. Cheson, MD, director of hematology services at Georgetown's Lombardi Cancer Center in Washington, DC.

"I've used this drug more times than I can count and it's exceptionally well tolerated," he says. "It's one of the major advances in hematology/oncology in my lifetime."