OBJECTIVE: To determine whether metformin is excreted into breast milk and whether this exposure adversely affects the blood glucose of nursing infants. METHODS: Seven women were started on metformin 500 mg twice daily on the first day after cesarean delivery. Breastfeeding was started at the same time. Two women were excluded. Two other women stopped breastfeeding for personal reasons unrelated to the drug therapy, but did provide serum and milk samples, because they regularly pumped their breasts to maintain lactation. Peak and trough serum and milk samples were drawn between postoperative days 4 and 17. In 3 infants, blood was drawn for glucose determination at the same time as the maternal samples. RESULTS: The trough milk concentration in 1 subject was below the assay detection limit. Excluding this subject, the mean peak and trough serum metformin concentrations were 1.06 μ g/mL (range 0.68- 1.90 μ g/mL) and 0.42 μ g/mL (range 0.26- 0.51 μ g/mL), respectively, whereas the mean peak and trough metformin concentrations in breast milk were 0.42 μ g/mL (range 0.38- 0.46 μ g/mL) and 0.39 μ g/mL (range 0.31- 0.52 μ g/mL), respectively. The mean milk:serum ratio was 0.63 (range 0.36- 1.00) and the mean estimated infant dose as a percentage of the mother’s weigh- adjusted dose was 0.65% (range 0.43- 1.08% ). In 3 infants, the blood glucose concentrations 4 hours after a feeding were within the normal limit, ranging from 47- 77 mg/dL. CONCLUSION: Metformin is excreted into breast milk, but the amounts seem to be clinically insignificant. No adverse effects on the blood glucose of the 3 nursing infants were measured. METHODS: Seven women were started on metformin 500 mg twice daily on the first day after cesarean delivery. Breastfeeding was started at the same Two women were unrelated to the drug therapy, but did provide serum and milk samples, because they regularly pumped their breasts to maintain lactation. Two and the women were unrelated to the drug therapy days 4 and 17. In 3 infants, blood was drawn for glucose determination at the same time as the maternal samples. RESULTS: The trough milk concentration in 1 subject was below the assay detection limit. metformin concentrations were 1.06 μg / mL (range 0.68-1.90 μg / mL) and 0.42 μg / mL (range 0.26-0.51 μg / mL), respectively, the mea n peak and trough metformin concentrations in breast milk were 0.42 μg / mL (range 0.38-0.46 μg / mL) and 0.39 μg / mL (range 0.31-0.52 μg / mL), respectively. The mean milk: serum ratio was 0.63 (range 0.36- 1.00) and the mean estimated infant dose as a percentage of the mother’s weigh- adjusted dose was 0.65% (range 0.43-1.08%). In 3 infants, the blood glucose concentrations 4 hours after a feeding were within the normal limit, ranging from 47-77 mg / dL. CONCLUSION: Metformin is excreted into breast milk, but the figures seem to be clinically insignificant. No adverse effects on the blood glucose of the 3 nursing infants were measured.