Is Metformin Safe for Breastfeeding?
Yes, metformin is safe for breastfeeding mothers with diabetes, as it is excreted into breast milk in clinically insignificant amounts with no adverse effects on nursing infants. 1
Guideline-Based Recommendations
The Fifth International Workshop-Conference on Gestational Diabetes explicitly states that metformin does not appear to have harmful neonatal effects in breastfeeding women, though they noted at the time that larger studies were needed. 2 The American Academy of Dermatology recommends metformin as safe during breastfeeding, particularly beneficial for patients with metabolic comorbidities. 1
For postpartum women with persistent diabetes after gestational diabetes or those with type 2 diabetes, metformin can be continued during breastfeeding as a first-line pharmacologic option. 1
Milk Transfer and Infant Exposure Data
The evidence demonstrates reassuring safety margins:
- The mean milk-to-plasma ratio for metformin is 0.35-0.63, indicating limited transfer into breast milk. 3, 4
- Infant exposure averages only 0.28-0.65% of the weight-adjusted maternal dose, well below the 10% threshold of concern for breastfeeding. 3, 4
- Metformin concentrations in breast milk average 0.27-0.42 mg/L, with peak milk concentrations remaining low even at maternal doses up to 1500 mg daily. 3, 4
- Metformin is present in very low or undetectable concentrations in infant plasma, and measured infant blood glucose levels remain normal (47-77 mg/dL). 3, 4
The FDA drug label confirms that metformin is present in human milk at levels resulting in infant doses of approximately 0.11-1% of the maternal weight-adjusted dosage, with a milk/plasma ratio between 0.13 and 1. 5
Clinical Safety Evidence
No adverse effects on blood glucose, growth, motor-social development, or intercurrent illness have been reported in nursing infants exposed to metformin through breast milk during the first 6 months of life. 6
Studies of breastfeeding infants whose mothers took metformin found no health problems in evaluated infants, supporting the conclusion that metformin use by breastfeeding mothers is safe. 3
Important Safety Caveats
While metformin is safe for breastfeeding, certain precautions apply:
- Temporarily discontinue metformin during acute illness, dehydration, or administration of iodinated contrast to avoid lactic acidosis. 1
- Monitor maternal vitamin B12 levels with long-term metformin use, as deficiency can occur. 1
- Metformin may restore ovulation in anovulatory women, increasing pregnancy risk—discuss contraception needs with all premenopausal breastfeeding patients starting metformin. 1
Practical Prescribing Approach
Metformin is considered a first-line choice for breastfeeding patients with type 2 diabetes, gestational diabetes, or polycystic ovary syndrome requiring pharmacologic therapy. 1
For women with persistent hyperglycemia after gestational diabetes, metformin can be safely initiated or continued during breastfeeding without concern for infant harm, though insulin, glyburide, or glipizide are also safe alternatives. 2
Common Pitfalls to Avoid
Do not unnecessarily avoid metformin in breastfeeding women due to unfounded concerns about infant exposure—the evidence clearly demonstrates safety with minimal drug transfer and no adverse infant outcomes. 3, 4, 6
The FDA label notes insufficient information exists to determine all effects on breastfed infants due to small sample sizes in published studies, but this reflects methodological limitations rather than safety concerns, as all available data support safety. 5