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OBJECTIVE: To report adverse effects in a newborn infant whose mother had been treated with doxepin during pregnancy and while breast-feeding. CASE SUMMARY: The nine-day-old white boy was admitted because of poor sucking and swallowing, with muscle hypotonia and vomiting. He was drowsy and had lost 150 g. At the time of admission, he was breast-fed by his mother who was being treated with doxepin 35 mg/d. Samples of plasma and breast milk were taken and analyzed by HPLC and fluorescence polarization immunoassay. The amount of doxepin and N-desmethyldoxepin (DDP) ingested via breast-feeding was approximately 10–20 μg/kg/d (i.e., only 2.5% of the weight-adjusted dose of the mother). Doxepin was detectable in small amounts in the infant's plasma (∼10 μg/L); DDP was below the lower limit of detection of 10 μg/L. All adverse effects subsided within 48 hours after breast-feeding was stopped. DISCUSSION: Despite the small doses of doxepin and its active metabolite ingested by breast-fed babies, there is a risk of accumulation and resultant adverse effects. In newborns, the metabolic activity is considerably decreased and may be further reduced by hyperbilirubinemia. CONCLUSIONS: Available data suggest that women treated with doxepin should breast-feed their infants with great caution, if at all, although much larger databases are needed to confirm this.