Fluoxetine and norfluoxetine concentrations in nursing infants and breast milk

Background: This study's goal was to characterize nursing infants' exposure to fluoxetine through breast milk and to identify variables for minimizing such exposure. Methods: Nursing women on stable daily doses of fluoxetine were recruited into the study. Breast milk, maternal and infant serum concentrations of fluoxetine and norfluoxetine were determined with high-performance liquid chromatography. Results: Nineteen nursing women (one with a pair of dizygotic twins) participated in the study. The women were on stable daily doses of fluoxetine (10-60 mg/day) and all but two took the medication during the last trimester of pregnancy. Fluoxetine was detectable in 30% (n = 6) of the nursing infant sera (< 1-84 ng/mL), whereas norfluoxetine was found in 85% (N = 17) (< 1-265 ng/mL). Peak breast milk concentrations occurred approximately 8 hours after maternal dosing and predicted norfluoxetine concentrations in infant serum. Maternal serum fluoxetine and norfluoxetine concentrations correlated highly with infant norfluoxetine concentrations. A daily maternal fluoxetine dosage of 20 mg or lower was significantly less likely to produce detectable concentrations of either fluoxetine or norfluoxetine in infants compared to higher daily dosages. No adverse effects were reported in any infant. Conclusions: Our findings demonstrate that maternal serum and peak breast milk concentrations of fluoxetine and norfluoxetine predict nursing infant serum norfluoxetine concentrations. In nursing women taking 20 mg/day or less of fluoxetine, infant serum concentrations were typically low.