In the treatment of conditions such as ulcerative colitis and Crohn's disease, most professional guidelines and other experts consider breastfeeding to be acceptable during mercaptopurine therapy.[1-9] Azathioprine is rapidly converted to mercaptopurine, so data from mothers taking azathioprine apply to mercaptopurine. No active metabolites of mercaptopurine were found in the blood of breastfed infants whose mothers were taking azathioprine and only poorly documented cases of mild, asymptomatic neutropenia and increased rates of infection have been reported occasionally. It might be desirable to monitor exclusively breastfed infants with a complete blood count with differential, and liver function tests if azathioprine is used during lactation, although some authors feel that such monitoring is unnecessary.[10]. See the Azathioprine record for details. Mothers with decreased activity of the enzyme that detoxifies mercaptopurine metabolites may transmit higher levels of drug to their infants in breastmilk. It might be desirable to monitor exclusively breastfed infants with a complete blood count with differential, and liver function tests if mercaptopurine is used during lactation, although some authors feel that monitoring is unnecessary.[11] Avoiding breastfeeding for 4 hours after a dose should markedly decrease the dose received by the infant in breastmilk.[12]
Most sources consider breastfeeding to be contraindicated during maternal antineoplastic drug therapy, although antimetabolites such as mercaptopurine appear to pose the least risk to breastfed infants.[13] After high-dose chemotherapy, it might be possible to breastfeed safely during intermittent therapy with an appropriate period of breastfeeding abstinence. Although no data are available to determine an appropriate period to withhold breastfeeding, the drug's terminal half-life suggests that withholding breastfeeding for 1 to 2 days may be sufficient. Chemotherapy may adversely affect the normal microbiome and chemical makeup of breastmilk.[14]