Is metformin (biguanide oral hypoglycemic agent) safe for use in lactating mothers?

Metformin Safety in Lactating Mothers

Metformin is generally safe for use in lactating mothers, with minimal infant exposure through breast milk and no reported adverse effects on nursing infants. 1, 2

Evidence on Metformin in Breast Milk

• Metformin is present in breast milk but at very low concentrations, with milk-to-plasma ratios averaging around 0.35 (95% CI 0.2-0.5) 2
• The mean infant exposure to metformin through breast milk is only 0.28% (range 0.16-0.4%) of the weight-normalized maternal dose, well below the 10% level of concern for breastfeeding medications 2
• Studies report infant exposure to metformin in the range of 0.28-1.08% of the maternal dose, which is considered minimal 3
• Metformin concentrations in infant plasma are very low or undetectable in most cases 2

Effects on Nursing Infants

• No adverse effects on blood glucose levels have been reported in nursing infants whose mothers take metformin 3
• Studies comparing breastfed infants of metformin-treated mothers to formula-fed infants found:
  • No differences in growth parameters (weight, height) at 3 and 6 months 4
  • No differences in motor-social development 4
  • No differences in intercurrent illness requiring pediatrician visits 4
  • No evidence of developmental retardation 4

FDA Labeling Information

• The FDA label for metformin states that limited published studies report metformin is present in human milk 1
• However, the label notes there is "insufficient information to determine the effects of metformin on the breastfed infant" 1
• The FDA recommends considering "the developmental and health benefits of breastfeeding" along with the mother's clinical need for metformin and any potential adverse effects 1

Clinical Considerations

• Metformin is often preferred over other oral antidiabetic agents during lactation due to its established safety profile 3, 5
• For mothers with type 2 diabetes, metformin may help control blood glucose while breastfeeding 5
• For mothers with polycystic ovary syndrome (PCOS), metformin may be continued during lactation with no apparent adverse effects on infants 3, 4

Limitations and Precautions

• While short-term safety data is reassuring, long-term follow-up data on infants exposed to metformin through breast milk is limited 5
• As with any medication during lactation, clinical monitoring of the infant is recommended, particularly for signs of hypoglycemia, although this appears to be rare 2
• Metformin should be used with caution in mothers with severe renal impairment due to potential drug accumulation 1

In conclusion, the available evidence supports that metformin use during lactation poses minimal risk to nursing infants, with no documented adverse effects on infant growth or development. The benefits of breastfeeding generally outweigh the theoretical risks of the minimal metformin exposure through breast milk.