Letrozole is approved for use only in postmenopausal women with breast cancer. But there is evidence some doctors have prescribed it as a fertility treatment because it suppresses estrogen and can promote ovulation. It is marketed under the name Femara.
Swiss drug manufacturer Novartis is sending letters to fertility doctors worldwide to reiterate a warning that the drug should not be given to women who may be pregnant, said spokeswoman Kim Fox.
The U.S. label on the drug already warns that it has been associated with birth defects, but concerns arose when a researcher in Canada published a report noting cases where the drug had been given to pregnant women.
After the report, Novartis reviewed its safety database and found 13 reports of pregnant women receiving the drug worldwide, contrary to its warning label.
Of those 13, at least two had miscarriages and two had children with birth defects, Fox said. It was not clear how many of these women were given the drug to increase their fertility as opposed to some other reason.
Health Canada, the country's health care agency, issued the warning jointly with Novartis to fertility specialists, gynecologists and obstetricians.
In its letter in Canada, Novartis said it is "aware that Femara is being used to stimulate ovulation in women who are infertile, or unable to become pregnant, as a treatment to increase their chances of becoming pregnant."
The drug "should not be used in women who may become pregnant, during pregnancy and/or while breast-feeding, because there is a potential risk of harm to the mother and the fetus, including risk of fetal malformations," the company said.
Pregnant women who have received the drug should contact their doctor.
The U.S. Food and Drug Administration has not taken any action, but a spokeswoman said the agency was reviewing the matter. Novartis plans to send a letter to U.S. fertility specialists by the end of the week, said Fox of Novartis.
It is unknown how many women have been prescribed the drug as a fertility treatment. Doctors in many specialties sometimes prescribe drugs for uses not approved by health regulators, believing the drug can still serve to treat a condition.