Contra-indications and warnings for azathioprine

• pregnancy and lactation - limited data suggests that if a woman becomes pregnant while taking a thiopurine for inflammatory bowel disease, continuing treatment does not increase the risks of pregnancy complications or congenital abnormalities. However, it seems prudent not to start a thiopurine if a woman plans to become, or is, pregnant, nor should women breast feed- while taking a thiopurine (1).
• gross renal or hepatic disease
• blood dyscrasias
• care with allopurinol. Reduce the azathioprine dose to one quarter of the original
• thiopurine S-methyl transferase (TPMT)
  • assessment of TPMT is useful in the treatment of patients with thiopurine drugs such as azathioprine, mercaptopurine and 6-thioguanine
    • 0.3 % of the population do not have this enzyme and can build up toxic levels of the active metabolites. Another 10% have a medium activity and may also need a lower dose than other patients
      • the strategy of determining TPMT activity in all patients prior to initiating treatment with azathioprine could help to minimize the risk of myelotoxicity, as patients with intermediate TPMT activity had fourfold more risk than high TPMT activity patients (2)

Check the summary of product characteristics for detailed information.

Reference:

1. Drug and Therapeutics Bulletin (2002), 39 (12), 91-95.
2. Gisbert JP et al. Thiopurine Methyltransferase (TPMT) Activity and Adverse Effects of Azathioprine in Inflammatory Bowel Disease: Long-Term Follow-Up Study of 394 Patients. Am J Gastroenterol. 2006 Oct 6