<p>Oral administration of letrozole in pregnant rats resulted in teratogenicity and maternal toxicity at 0.03 mg/kg. Embryotoxicity and foetotoxicity were seen at doses &gt;0.003 mg/kg and there was an increase in the incidence of foetal malformation among the animals treated. However, there are no adequate and well-controlled studies of letrozole in pregnant women and its use in these patients is not recommended. It is not known whether letrozole is excreted in human milk. Because many drugs are excreted in human milk, letrozole should not be administered to a nursing woman.</p>