Among Michigan Medicaid recipients, 1617 exposures to clemastine were reported during the first trimester. (F. Rosa, personal communication, FDA, 1994) Birth defects were observed in 71 infants (68 expected) and included 13 cardiovascular defects, 3 oral clefts, 3 spina bifida, 4 polydactyly, 4 hypospadias, and 5 limb reductions. No evidence was found to suggest a relationship to large categories of malformations, except possibly limb reductions.

A review of prenatal drug use in 3026 women with premature infants demonstrated an increased risk of retrolental fibroplasia with antihistamine use during the last two weeks of pregnancy. The dosage used or the particular antihistamine was not specified. The incidence of retrolental fibroplasia in premature infants exposed in utero to antihistamine during this time was 21% compared to 11% in premature infants not exposed.

Clemastine has been assigned to pregnancy category B by the FDA. There are no controlled data in human pregnancy. Clemastine is only recommended for use during pregnancy when benefit outweighs risk.

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Clemastine is excreted into breast milk. One breast-fed infant developed drowsiness, irritability, refusal to feed, neck stiffness, and a high-pitched cry 12 hours after her mother began clemastine, 1 mg twice daily. The mother was also receiving carbamazepine and phenytoin. The infant returned to normal after clemastine was discontinued. Twenty hours after a dose of clemastine, the mother's serum and breast milk concentrations were 20 mcg/mL and 5 to 10 mcg/mL, respectively. The American Academy of Pediatrics recommends that clemastine be used with caution during breast-feeding. The manufacturer recommends that due to the potential adverse reactions in nursing infants, a decision should be made to discontinue nursing or discontinue the drug, taking into account the importance of the drug to the mother.

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