Aims/hypothesis. The aim of this study was to characterize the milk-to-plasma ratio and infant dose for metformin in breastfeeding women, and to measure plasma concentrations and assess any effects in their infants. We hypothesized that metformin used by mothers is safe for their breastfed infants.

Methods. Seven women taking metformin (median dose 1500 mg orally daily) and their infants were studied. Metformin concentrations in plasma and milk were measured by high performance liquid chromatography. Infant exposure was estimated as the product of estimated milk production rate and the average concentration of the drug in milk and also expressed as a percentage of the weight-normalized maternal dose.

Results. The mean milk-to-plasma ratio for metformin was 0.35 (95%CI 0.2–0.5). The mean of its average concentrations in milk over the dose interval was 0.27 mg/l (0.15–0.39 mg/l). The absolute infant dose averaged 0.04 mg·kg^–1·day^–1 (0.02–0.06 mg·kg^–1·day^–1) and the mean relative infant dose was 0.28% (0.16–0.4%). Metformin was present in very low or undetectable concentrations in the plasma of four of the infants who were studied. No health problems were found in the six infants who were evaluated.

Conclusions/interpretation. The concentrations of metformin in breast milk were generally low and the mean infant exposure to the drug was only 0.28% of the weight-normalized maternal dose. As this is well below the 10% level of concern for breastfeeding, and because the infants were healthy, we conclude that metformin use by breastfeeding mothers is safe. Nevertheless, each decision to breastfeed should be made after conducting a risk:benefit analysis for each mother and her infant.