Is metformin (biguanide oral hypoglycemic agent) safe to use in breastfeeding patients?

Metformin Use During Breastfeeding

Metformin is safe to use during breastfeeding, with minimal transfer into breast milk and no clinically significant adverse effects on nursing infants. 1, 2

Evidence Supporting Safety

Transfer into Breast Milk

• Metformin is present in breast milk at very low concentrations, with a mean milk-to-plasma ratio of 0.35-0.63 3, 4
• The absolute infant dose averages only 0.04 mg/kg/day, representing 0.28-1.08% of the weight-normalized maternal dose 3, 4, 5
• This exposure is well below the 10% threshold of concern for breastfeeding safety 4

Clinical Outcomes in Infants

• Blood glucose levels in breastfed infants remain normal (47-77 mg/dL measured 4 hours after feeding) 3
• No adverse effects on infant growth, motor-social development, or intercurrent illness have been reported during the first 6 months of life 5
• Metformin concentrations in infant plasma are very low or undetectable 4

Guideline Recommendations

Specific Clinical Contexts

• Hidradenitis suppurativa: The American Academy of Dermatology recommends metformin as safe during breastfeeding, particularly beneficial for patients with metabolic comorbidities 1
• Gestational diabetes: The Fifth International Workshop-Conference on Gestational Diabetes states that limited data suggest metformin does not appear to have harmful neonatal effects, though larger studies were recommended at that time 1
• Type 2 diabetes: Patients with type 2 diabetes can continue metformin during breastfeeding 1

FDA Labeling

• The FDA label acknowledges that metformin is present in human milk at infant doses of approximately 0.11-1% of maternal weight-adjusted dosage 2
• The label emphasizes weighing developmental and health benefits of breastfeeding against the mother's clinical need for metformin 2

Practical Prescribing Approach

When to Use Metformin

• First-line choice for breastfeeding patients with type 2 diabetes, gestational diabetes, or polycystic ovary syndrome requiring pharmacologic therapy 1, 6
• Preferred over sulfonylureas due to lower hypoglycemia risk and weight neutrality 6
• Can be initiated or continued postpartum without interrupting breastfeeding 1

Dosing Considerations

• Use standard adult dosing (typically 500-2550 mg daily in divided doses) 2
• Start with low doses and titrate gradually to minimize maternal gastrointestinal side effects 7
• Take with meals to reduce GI symptoms 7

Important Caveats

Maternal Contraindications Still Apply

• Do not use metformin in breastfeeding patients with severe renal impairment (eGFR <30 mL/min/1.73 m²) 2
• Avoid in patients with hepatic impairment, severe heart failure, or conditions predisposing to lactic acidosis 2
• Temporarily discontinue during acute illness, dehydration, or administration of iodinated contrast 1, 2

Monitoring Recommendations

• Monitor maternal vitamin B12 levels with long-term use, as deficiency can occur and potentially affect milk quality 8, 9
• Assess maternal renal function periodically, especially in older patients 2
• No specific infant monitoring is required beyond routine well-child care 3, 4

Common Pitfall to Avoid

• Do not unnecessarily discontinue metformin in breastfeeding patients due to unfounded safety concerns—the evidence consistently demonstrates safety 1, 3, 4, 5
• Do not confuse metformin safety during breastfeeding with concerns about its use during pregnancy (which requires separate risk-benefit analysis) 1, 2

Contraception Counseling

• Metformin may restore ovulation in anovulatory women, increasing pregnancy risk 2
• Discuss contraception needs with all premenopausal breastfeeding patients starting metformin 2
• Estrogen-containing contraceptives can be initiated 6-8 weeks postpartum without affecting metformin safety 1