The Food and Drug Administration warned one more time about the risks of Novartis and Roche transplant drugs for pregnant woman.

On Friday, the U.S. health regulator referred to Roche's drug CellCept and Novartis AG' drug Myfortic on a warning letter. In October both were linked to reports of miscarriages and birth defects when prescribed to pregnant women.

In November the companies changed the labels adding a strong warning about the risks. The FDA said it hasn't received more reports of pregnancy related problems but said the letter was a reminder to doctors who prescribe the drugs.

The agency encourages doctors to make sure their organ transplant patients are not pregnant before prescribing the medicine and also a reminder for the prescription for the approved use of preventing rejection of transplanted organs.

The FDA said it will continue working with both companies to halt use of the products by pregnant women.

Shares of Novartis were lastly trading 0.10 percent down to $51.36 in the New York Mercantile Exchange in after hours electronic trading Friday.