The U.S. Food and Drug Administration on Friday warned doctors about a certain type of anemia in patients being treated with the Roche (ROG.VX) kidney-transplant drug Cellcept.

The FDA posted a letter from Roche to health-care professionals to its Web site on Friday discussing the problem. The FDA said Cellcept's label has been revised to warn doctors about cases of pure red cell aplasia.

Last month, the FDA said it was requiring Roche and other manufacturers of certain drugs used to prevent the rejection of transplanted kidneys to warn of certain serious infections.

The drugs, including Cellcept, Myfortic by Novartis AG (NVS) and Wyeth's (WYE) Rapamune, already carry the agency's toughest boxed warning discussing their various risks.

The FDA said the drug labels must discuss an increased risk of "opportunistic infections," including activation of latent viral infections such as one caused by the BK virus.

Last year, Novartis and Roche updated the labels of Myfortic and Cellcept to discuss postmarketing reports of a serious, often deadly neurological disease known as progressive multifocal leukoencephalopathy, or PML. The disease is caused by the activation of a common virus in the central nervous system, most often in immune-compromised individuals. The virus, known as the JC virus, sits latent in most adults.

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