Clemastine and Pregnancy
Based on animal studies on pregnancy and clemastine, this medication seems to be relatively safe for use during pregnancy. Although antihistamines are often used off-label to treat severe morning sickness, they should not be taken during the last two weeks of pregnancy. If you are taking this medicine and pregnancy occurs, be sure to talk to your healthcare provider about any potential risks.
Clemastine fumarate (Tavist® Allergy, Dayhist® Allergy) is an antihistamine medication. Sometimes, antihistamines like clemastine are used "off-label" for the treatment of severe morning sickness. Based on animal studies, this medication seems to be relatively safe for use during pregnancy, although the full effects in humans are not currently known.
The U.S. Food and Drug Administration (FDA) uses a pregnancy category system to classify the possible risks to a fetus when a specific medicine is taken during pregnancy. Pregnancy Category B is given to medicines that have not been adequately studied in pregnant humans but do not appear to cause harm to the fetus in animal studies. Medications that have been shown to be safe for use in pregnancy in humans (but have caused problems in laboratory animals) are also given a Category B rating.
In studies, giving very large doses of clemastine to pregnant rabbits and rats did not increase the risks of any problems. In human studies (which have not been adequate enough to fully characterize the safety of the medication), antihistamines seem to be fairly safe for use during pregnancy. It should be noted that the use of antihistamines in general during the last two weeks of pregnancy seems to increase the risk of retinopathy of prematurity (an eye problem in premature infants that can lead to blindness).