Breastfeeding during phenytoin monotherapy does not appear to adversely affect infant growth or development, and breastfed infants had higher IQs and enhanced verbal abilities than nonbreastfed infants at 6 years of age in one study.[1] If phenytoin is required by the mother, it is not a reason to discontinue breastfeeding.
Because of the low levels of phenytoin in breastmilk, amounts ingested by the infant are small and usually cause no difficulties in breastfed infants when used alone except for rare idiosyncratic reactions. Combination therapy with sedating anticonvulsants or psychotropics may result in infant sedation or withdrawal reactions. In one case report, maternal phenytoin dosage requirements decreased as breastfeeding was discontinued.[2]