The Food and Drug Administration Thursday warned patients on the psoriasis drug Raptiva about the risk of developing a rare and often fatal brain infection known as progressive multifocal leukoencephalopathy.

In a public health advisory posted on the agency's Web site, the FDA said it's received three confirmed and one possible report of PML in patients taking Raptiva since the drug was approved in 2003. Raptiva is marketed by Genentech (DNA) and is approved to treat a condition characterized by inflamed, swollen and scaly patches of skin. The agency said three of the patients died.

Last October the product labeling for Raptiva was revised to highlight in a boxed warning the risks of life-threatening infections, including PML, after one case of PML was seen.

The FDA said it is reviewing the additional cases of PML "and will take appropriate steps to ensure that the risks of Raptiva do not outweigh its benefits."

-By Jennifer Corbett Dooren, Dow Jones Newswires; 202-862-9294; jennifer.corbett@dowjones.com.