A drug to prevent organ rejection in transplant patients may cause birth defects, health officials warned Monday.The drug, called CellCept, also may cause miscarriages, the Food and Drug Administration added. The FDA also warned that taking the drug, made by Switzerland's Roche Holding, may lower blood levels of the hormones in birth control pills, theoretically making them less effective at preventing pregnancy.

Women of childbearing potential should have a negative pregnancy test within a week of starting the immune-system suppressing drug, and must receive birth control counseling and use effective contraception, the FDA said.

Some doctors prescribe CellCept to lupus patients who cannot tolerate chemotherapy. The drug is known generically as mycophenolate mofetil.