Is metformin (metformin hydrochloride) safe during lactation (breast feeding)?

From the Guidelines

Metformin is generally considered safe during breastfeeding, with the most recent and highest quality evidence supporting its use in nursing mothers with diabetes or other conditions requiring metformin therapy. The amount of metformin that passes into breast milk is very small, with infants receiving less than 1% of the mother's weight-adjusted dose, which is unlikely to cause any adverse effects in the breastfed infant 1. No significant problems have been reported in babies whose mothers take metformin while breastfeeding.

Key Considerations

• The benefits of continued breastfeeding while on metformin typically outweigh any theoretical risks, given the numerous health benefits of breastfeeding for both mother and child, including reduced infant mortality, respiratory infections, and asthma, as well as improved maternal health outcomes such as reduced risks of breast cancer, ovarian cancer, and type 2 diabetes 1.
• However, premature infants or those with kidney problems may need additional consideration due to potentially reduced drug clearance.
• As with any medication during lactation, it's advisable to monitor the infant for unusual symptoms such as poor feeding, excessive sleepiness, or digestive issues.
• If metformin is needed for managing diabetes, maintaining good glucose control is important for both maternal health and successful lactation, and breastfeeding may also confer longer-term metabolic benefits to both mother and offspring 1.

Evidence Summary

• A 2025 study published in Diabetes Care found that breastfeeding was associated with numerous health benefits for children and improved maternal health outcomes, and that metformin can be safely used by breastfeeding women 1.
• An earlier study from 2007 also suggested that metformin, while excreted into breast milk, does not appear to have harmful neonatal effects, although larger studies were needed to demonstrate safety 1. However, given the more recent and higher quality evidence from 2025, metformin is considered safe for use during lactation.

From the FDA Drug Label

Limited published studies report that metformin is present in human milk [see Data] . However, there is insufficient information to determine the effects of metformin on the breastfed infant and no available information on the effects of metformin on milk production Therefore, the developmental and health benefits of breastfeeding should be considered along with the mother's clinical need for metformin hydrochloride tablets and any potential adverse effects on the breastfed child from metformin hydrochloride tablets or from the underlying maternal condition Published clinical lactation studies report that metformin is present in human milk which resulted in infant doses approximately 0.11 % to 1% of the maternal weight-adjusted dosage and a milk/plasma ratio ranging between 0. 13 and 1.

The safety of metformin during lactation is not established due to insufficient information on its effects on the breastfed infant and milk production. The decision to use metformin during breastfeeding should weigh the developmental and health benefits of breastfeeding against the mother's clinical need for metformin and potential adverse effects on the infant 2.

From the Research

Safety of Metformin during Lactation

• Metformin is considered safe for use during breastfeeding, with several studies indicating that it does not harm the infant 3, 4.
• The concentration of metformin in breast milk is generally low, with a mean milk-to-plasma ratio of 0.35 3.
• The estimated infant dose of metformin is approximately 0.28% of the weight-normalized maternal dose, which is well below the level of concern for breastfeeding 3.
• No adverse effects on the blood glucose levels of nursing infants have been reported, and metformin use during lactation does not appear to affect infants' growth, motor-social development, or intercurrent illness 4.
• Other studies have also suggested that metformin is a rational alternative to insulin therapy during pregnancy and breastfeeding, with no serious safety concerns 5, 6.
• According to a review of medication safety during breastfeeding, metformin is generally preferred for treating diabetes mellitus during breastfeeding, along with insulin and second-generation sulfonylureas 7.