Drug Levels and Effects:

Summary of Use during Lactation:

Most sources consider breastfeeding to be contraindicated during maternal antineoplastic drug therapy. In the treatment of conditions such as ulcerative colitis and Crohn's disease, low doses of mercaptopurine (6-MP), equivalent to those generated by the use of azathioprine for immunosuppression, appear to be acceptable.[1][2][3] No active metabolites of azathioprine were found in the blood of breastfed infants whose mothers were taking azathioprine and no adverse effects attributable to azathioprine have been noted. See 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 azathioprine is used during lactation, although some authors feel that monitoring is unnecessary.[4]

Drug Levels:

Mercaptopurine is the active metabolite of azathioprine. It is further metabolized to active metabolites including 6-methylmercaptopurine, thioguanine, 6-thioguanine nucleoside (6-TGN) and 6-methylmercaptopurine nucleosides (6-MMPN). The enzyme thiopurine methyltransferase (TPMT) is responsible for metabolism of 6-TGN. Deficiencies in this enzyme can lead to excessive toxicity.

Maternal Levels.

Mercaptopurine milk levels were measured in 2 patients receiving azathioprine following renal transplantation. In one, peak milk levels occurred 2 and 8 hours after a 75 mg oral dose and were 3.4 and 4.5 mcg/L respectively. In the other, a peak milk level of 18 mcg/L occurred 2 hours after a 25 mg oral dose. Serum levels were not measured.[5]

Four women receiving an immunomodulator to treat inflammatory bowel disease had metabolite levels measured in milk during the first 6 weeks postpartum. The abstract does not mention the specific drug and dose being taken, but the azathioprine metabolites 6-methylmercaptopurine (6-MMP) and 6-thioguanine nucleoside (6-TGn) were measured. Although therapeutic levels were found in maternal serum, 6-MMP (<650 mcg/L) and 6-TGn (<123 mcg/L) were undetectable in milk (time of collection not stated.[6]

Infant Levels.

Four infants were breastfed (3 exclusively, 1 rarely received formula) during maternal use of azathioprine orally in dosages of 1.2 to 2.1 mg/kg daily. All of the mothers and infants had the normal TPMT *1/*1 genotype and all of the mothers had normal enzyme activity. At 3 to 3.5 months of age, all of the infants' had undetectable blood levels of 6-TGN and 6-MMPN.[7]

Effects in Breastfed Infants:

Relevant published information was not found as of the revision date; however, mercaptopurine is the active metabolite of azathioprine. At least 60 infants breastfed during maternal azathioprine in dosages up to 250 mg daily have been reported with no adverse effects noted. See Azathioprine record for details.

Possible Effects on Lactation:

Relevant published information was not found as of the revision date.

Alternate Drugs to Consider:

(Immunosuppression) Cyclosporine,Tacrolimus,(Inflammatory Bowel Disease) Budesonide,Infliximab,Mesalamine,Prednisone,(Systemic Lupus Erythematosus) Hydroxychloroquine,Prednisone


1. Ha C, Dassopoulos T. Thiopurine therapy in inflammatory bowel disease. Expert Rev Gastroenterol Hepatol. 2010;4 :575-88. PMID:20932143
2. van der Woude CJ, Kolacek S, Dotan I et al. European evidenced-based consensus on reproduction in inflammatory bowel disease. J Crohn's Colitis. 2010;4:493-510. PMID:21122553
3. Van Assche G, Dignass A, Reinisch W et al. The second European evidence-based Consensus on the diagnosis and management of Crohn's disease: Special situations. J Crohns Colitis. 2010;4:63-101. PMID:21122490
4. Christensen LA, Dahlerup JF, Nielsen MJ et al. Azathioprine treatment during lactation: authors' reply. Aliment Pharmacol Ther. 2009;30:91. PMID:19566905
5. Coulam CB, Moyer TP, Jiang NS et al. Breast-feeding after renal transplantation. Transplant Proc. 1982;13:605-9. PMID:6817481
6. Kane SV, Present DH. Metabolites to immunomodulators are not detected in breast milk . Am J Gastroenterol. 2004;99(10 Suppl. S):S246-7. Abstract 761.
7. Gardiner SJ, Gearry RB, Roberts RL, Zhang M, Barclay ML, Begg EJ. Exposure to thiopurine drugs through breast milk is low based on metabolite concentrations in mother-infant pairs. Br J Clin Pharmacol. 2006;62:453-6. PMID:16995866

Substance Identification:

Substance Name:


CAS Registry Number:


Drug Class:

  • Antineoplastic Agents

  • Administrative Information:

    LactMed Record Number:


    Last Revision Date:

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