Mexiletine has been assigned to pregnancy category C by the FDA. Animal studies have failed to reveal evidence of teratogenicity or impaired fertility, but increased fetal resorption has been observed following dosages up to four times the maximum recommended human dose (on a per kg basis). There are no controlled data in human pregnancy. Mexiletine should be given during pregnancy only when benefit outweighs risk.

There is a paucity of data on the use of mexiletine during human pregnancy. In the limited data available, there is no evidence of teratogenicity or fetotoxicity associated with mexiletine. Mexiletine crosses the human placenta. In the first case described below, the umbilical cord to maternal plasma mexiletine levels were each 0.3 mcg per mL.

Mexiletine 200 mg and propranolol 40 mg three times a day were given to a 34-year-old woman from gestation week 32 through delivery at week 39 for paroxysmal ventricular tachycardia. At delivery, the infant was structurally normal, but suffered bradycardia (90 beats per min), which was thought to be due to propranolol. The infant's heart rate spontaneously increased to normal.

No adverse effects on the fetus or newborn were noted after mexiletine 200 mg three times a day and propranolol or atenolol were administered to two pregnant women, one of whom was taking mexiletine throughout pregnancy, and one of whom received it from gestation week 14 on. The newborn whose mother had taken mexiletine throughout pregnancy developed a seizure disorder at eight months of age, but had not been breast-fed since age three months.

In general, due to the lack of experience with mexiletine during human pregnancy, complete safety information is unavailable.

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Limited data reveal that mexiletine typically concentrates in human milk. Paired milk and maternal serum mexiletine levels from a woman who was taking mexiletine 200 mg three times a day reveal a mean ratio of 1.5, with a peak milk concentration of 0.96 mcg per mL. In this report, as well as two others, no adverse effects on the nursing infant were observed.

Mexiletine is excreted into and concentrates in human milk. Limited data suggest that adverse effects in the nursing infant are unlikely. The manufacturer recommends an alternative method of infant feeding when mexiletine is considered necessary for the nursing woman. However, mexiletine is considered compatible with breast-feeding by the American Academy of Pediatrics.

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