In: Drugs and Lactation Database (LactMed®) [Internet]. Bethesda (MD): National Institute of Child Health and Human Development; 2006.


Thioguanine nucleosides are active intracellular metabolites of azathioprine and have been measured in breastmilk and in infant serum following maternal use of azathioprine as an immunosuppressant. Thioguanine levels in milk are low after immunosuppressant doses of the drug or of azathioprine. It might be desirable to monitor exclusively breastfed infants with a complete blood count with differential, and liver function tests if thioguanine is used during lactation, although some authors feel that monitoring is unnecessary.[1] Most sources consider the thiopurines acceptable to use in low, immunosuppressant doses.[2-5]

Most sources consider breastfeeding to be contraindicated during maternal antineoplastic drug therapy, although antimetabolites such as thioguanine appear to pose the least risk to breastfed infants.[6] After high-dose thioguanine use in 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 4 days may be sufficient. Chemotherapy may adversely affect the normal microbiome and chemical makeup of breastmilk.[7]

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