Is treatment with metformin considered compatible with breast-feeding?/nThe question concerns a wom

Fråga: Is treatment with metformin considered compatible with breast-feeding?

The question concerns a woman with type II diabetes mellitus. During the pregnancy she received insulin treatment, but now when she is newly delivered the physician wants to continue her metformin treatment if possible. In FASS (The Swedish catalogue of approved medical products) there is no information about metformin transfer into human milk.

Sammanfattning: Metformin appears to be compatible with breast-feeding. However, data are limited and effects of continued exposure to small amounts of metformin on developing infants remains unknown.

Svar: Metformin is an oral biguanide antihyperglycemic agent used for treatment of type II diabetes mellitus (1). The prevalence of type II diabetes has increased by 30% in the past decade especially among younger persons, including pregnant and breast-feeding women (2).

The physicochemical properties of metformin give high water solubility and very low lipid solubility. It is incompletely absorbed from the gastro-intestinal tract (bioavailability 50-60%) and it does not give risk to hypoglycaemia in diabetic nor healthy patients (2).

Two studies describing metformin transfer into human milk were found in the literature (1,2). One of the studies included seven breast-feeding women taking oral doses of 1500 mg metformin a day in median (2). The other study included three breast-feeding women taking oral doses of 1000 mg metformin a day (1). In the two studies metformin concentration in the mothers´ milk and the infants´ plasma were measured at steady state. Metformin was present in very low or undetectable concentrations in the infants´ plasma. The infants weight adjusted doses of metformin were 0.28% and 0.20% respectively. None of the infants in the studies experienced any adverse effects. Both studies stated that metformin appears to be safe during breast-feeding (1,2).

The effects of continued exposure to very small amounts of metformin on developing infants remains unknown. Each decision to breastfeed should be made after a risk-benefit analysis for the mother and her infant. If metformin is used during breast-feeding the infant should be monitored for adverse effects such as altered feeding habits, diarrhea or failure to progress (1). A particular caution should be made when the infants´ renal function is low because metformin is largely excreted (90%) via the kidneys (2). Gardiner SJ, Kirkpatrick CM, Begg EJ, Zhang M, Moore MP, Saville DJ. Transfer of metformin into human milk. Clin Pharmacol Ther 2003;73:71-7. Hale TW, Kristensen JH, Hackett LP, Kohan R, Ilett KF. Transfer of metformin into human milk. Diabetologia 2002;45:1509-14.