Background  Thiopurines are widely used to maintain remission in inflammatory bowel disease. Treatment during pregnancy is generally recommended to improve the chance of a normal birth outcome, but advice concerning breastfeeding is conflicting.

Aim  To estimate the exposure of breastfed infants to 6-mercaptopurine, as a metabolite of azathioprine, from maternal milk.

Methods  Eight lactating women with inflammatory bowel disease receiving maintenance therapy with azathioprine 75–200 mg daily were studied. Milk and plasma samples were obtained 30 and 60 min after drug administration and hourly for the following 5 h.

Results  The variation in the bioavailability of the drug was reflected in a wide range of peak plasma values of 6-mercaptopurine within the first 3 h. A similar curve, but with an hour’s delay and at significantly lower concentrations varying from 2–50 μg/L, was seen in maternal milk. After 6 h an average of 10% of the peak values were measured.

Conclusions  The major part of 6-mercaptopurine in breast milk is excreted within the first 4 h after drug intake. On the basis of maximum concentration measured, the infant ingests mercaptopurine of <0.008 mg/kg bodyweight/24 h. The findings confirm that breastfeeding during treatment with azathioprine seems safe and should be recommended, considering the extensive beneficial effects.