Health regulators reiterated their warning Friday that organ transplant drugs from Roche and Novartis can cause miscarriages and birth defects when used by pregnant women.

The Food and Drug Administration previously said it received reports of miscarriages and infants born with ear and mouth birth defects after their mothers took Roche’s CellCept.

FDA added its most serious warning to CellCept and a similar Novartis AG drug, Myfortic, last October, noting they can cause miscarriage and other serious problems when used by pregnant mothers. The drug is used to suppress the body’s immune system to avoid organ rejection in transplant patients.

In a notice to physicians posted online Friday, FDA recommended “confirming that a pregnancy has not occurred and ensuring that patients use effective contraceptive measures.”

FDA said most of the reported problems came from mothers who were taking the drug before their pregnancies were detected. Some of the patients were taking the drugs for conditions they are not approved to treat _ including rheumatoid arthritis and lupus.

A spokesman for Roche said it has not received any new reports of miscarriages or birth defects since updating the drug’s labeling. The company previously reported 25 miscarriages among 77 women exposed to the drug between 1995 and 2007.

The agency said it will continue working with Roche and Novartis to reduce use of the drugs by pregnant women.

Friday’s FDA warning was the second in less than two months for CellCept, which was Roche’s six best-selling drug last year with sales of nearly $2 billion.

Last month FDA said it was investigating 16 patients who developed a rare neurological problems while taking the drug.

The disease, known as progressive multifocal leukoencephalopathy, attacks the brain and central nervous system and is usually fatal. Symptoms include vision problems, loss of coordination and memory loss. Patients who survive are often permanently disabled, according to the FDA.

Shares of Novartis AG rose 9 cents Friday to $51.29.