Metformin Use During Breastfeeding
Metformin is safe to use during breastfeeding and is recommended as a first-line agent for lactating women with diabetes mellitus, gestational diabetes, polycystic ovary syndrome, or metabolic comorbidities. 1, 2
Evidence Supporting Safety
The safety of metformin during lactation is well-established across multiple high-quality guidelines and research studies:
The American Academy of Dermatology (2025) explicitly recommends metformin as safe during breastfeeding, particularly beneficial for patients with metabolic comorbidities such as those with hidradenitis suppurativa. 1
The American College of Obstetricians and Gynecologists supports continuation of metformin during breastfeeding for patients with type 2 diabetes. 2
The FDA drug label acknowledges that metformin is present in human milk but notes that infant exposure is minimal, ranging from 0.11% to 1% of the maternal weight-adjusted dosage, with milk-to-plasma ratios between 0.13 and 1.0. 3
Pharmacokinetic Data
The transfer of metformin into breast milk has been well-characterized:
Mean milk-to-plasma ratio is 0.35-0.63, indicating limited transfer into breast milk. 4, 5
Absolute infant dose averages only 0.04 mg/kg/day, representing 0.28-0.65% of the weight-normalized maternal dose—well below the 10% threshold of concern for breastfeeding safety. 4, 5
Metformin concentrations in infant plasma are very low or undetectable, and no adverse effects on infant blood glucose have been documented. 4, 5
Clinical Outcomes in Breastfed Infants
Research demonstrates reassuring safety data:
No adverse effects on infant growth, motor-social development, or intercurrent illness have been reported during the first 6 months of life in infants exposed to metformin through breast milk. 6
Infant blood glucose levels remain within normal limits (47-77 mg/dL) even 4 hours after feeding from mothers taking metformin. 5
No health problems were identified in infants whose mothers used metformin while breastfeeding. 4
Important Clinical Caveats
While metformin is safe, certain precautions apply:
Temporarily discontinue metformin during acute maternal illness, dehydration, or administration of iodinated contrast to avoid risk of lactic acidosis. 2
Monitor maternal vitamin B12 levels with long-term metformin use, as deficiency can occur and potentially affect milk quality. 2
Counsel premenopausal breastfeeding women about contraception needs, as metformin may restore ovulation in anovulatory women, increasing pregnancy risk. Estrogen-containing contraceptives can be safely initiated 6-8 weeks postpartum without affecting metformin safety. 1, 2
Historical Context and Evolution of Evidence
The 2007 Fifth International Workshop-Conference on Gestational Diabetes noted that while limited data suggested metformin does not appear to have harmful neonatal effects, larger studies were needed at that time. 1 Since then, substantial evidence has accumulated, leading to current strong recommendations supporting its safety. The 2025 guidelines from the American Academy of Dermatology and summaries from multiple professional societies now confidently endorse metformin use during lactation. 1, 2