Maternal doses of procainamide 2 grams daily produced low levels of the drug and its active metabolite in the milk of one mother. Although it would not be expected to cause adverse effects in older breastfed infants, the relative lack of data concerning breastfeeding during maternal procainamide therapy would argue for careful monitoring if this drug is used while breastfeeding a neonate possibly Measurement of infant serum levels could help to rule out toxicity if there is a concern.