Mirtazapine Pregnancy and Breastfeeding Warnings
Mirtazapine Pregnancy Warnings
Mirtazapine has been assigned to pregnancy category C by the FDA. Studies in animals have revealed no indication of teratogenic effects. At twenty times the maximum recommended human dose, an increase was seen in rat pup deaths during the first three days of lactation and a decrease in birth weight. This effect was not seen at three times the maximum recommended human dose. In a study involving human pregnancy, mirtazapine was not associated with an increased risk of major malformation in neonates. Mirtazapine should only be given during pregnancy when benefit outweighs risk.
The results of a prospective, comparative, observational study (n=312) indicate that mirtazapine (mean daily dose 30 mg) does not appear to be associated with an increased risk of major malformation in newborns. In this study, 95% of the pregnant women were exposed to mirtazapine during the organogenesis period and 25% were exposed throughout pregnancy. However, exposure to mirtazapine was associated with a higher rate of preterm births.
Mirtazapine Breastfeeding Warnings
Following single daily doses of mirtazapine (median 38 mg; range 30 to 120 mg daily), the mean relative infant (n=8) doses for mirtazapine and desmethylmirtazapine were 1.5% and 0.4%, respectively, of the weight-adjusted maternal dose. Compared with the mean maternal dose of 495 mcg/kg/day, the mean absolute infant dose of mirtazapine was 8 mcg/kg/day. The mean milk/plasma ratios for mirtazapine and desmethylmirtazapine were 1.1 and 0.6, respectively. Among four infants tested, mirtazapine was detectable in plasma from only one infant (1.5 mcg/L) and desmethylmirtazapine was undetectable in all samples. No mirtazapine-related adverse effects were reported in any infant. A case report involving a 35-year-old mother receiving 22.5 mg per day of mirtazapine showed the weight-adjusted maternal dose of mirtazapine ranged from 3.9% to 4.4% at 4 hours postdose and 1.8% to 2.7 % at 10 hours postdose. Infant plasma levels were not detected 12 hours postdose.
Mirtazapine is excreted into human milk. Adverse effects in the nursing infant are unlikely; however, caution is advised when administering mirtazapine to nursing women with consideration given to risk/benefit.
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