Excretion of fluoxetine and its metabolite, norfluoxetine, in human breast milk

A study was conducted to measure breast milk concentrations of fluoxetine and its active metabolite, norfluoxetine, excreted in breast milk in a cohort of nursing women using fluoxetine, and to estimate infant dose from nursing. The study included 10 women nursing 11 infants (median age, 185 days). The mean fluoxetine dose was 0.39 mg/kg/day. Each patient manually collected 3 to 6 milk samples throughout a dosing interval. Concentrations of fluoxetine and norfluoxetine in milk were measured by gas-liquid chromatography. Mothers reported whether they observed adverse effects in their infants. The average infant doses of fluoxetine and norfluoxetine, as estimated for an exclusively breast-fed infant ingesting 1000 mL of milk per day, were 0.077 mg (SD = 0.054 mg) and 0.084 mg (SD = 0.043 mg), respectively. The total dose of fluoxetine and norfluoxetine (expressed as fluoxetine equivalents) was 0.165 mg (SD = 0.092 mg), which was equivalent to 10.8% (SD = 2.2%) of the maternal dose, adjusted on a mg/kg basis in a 4-kg infant. No adverse events were reported by mothers in their infants. Approximately one tenth of the adult therapeutic dose of fluoxetine is excreted in breast milk. Although short- term adverse effects in the infant from exposure through nursing were not reported in this cohort, future studies that assess the potentiated long- term consequences are needed.