Summary: Azathioprine (AZA) and 6-mercaptopurine (6-MP) belong to the thiopurine group. These are immunosuppressive and cytotoxic drugs. In inflammatory bowel diseases (IBD) they are used to support remission, both in ulcerative colitis and in Lesniowski--Crohn’s disease. Their main task is to replace glucocorticoids, which due to the amount of side effects cannot be administered in long-term therapy. The full operation of AZA can be observed only after about three months. Side effects occur in about 10-15% of patients with IBD. Between 30% and 50% of patients discontinue treatment and this is due to the lack of clinical response to treatment or side effects. The paper discusses the main methods of treatment optimization, which include: determining thiopurine methyltransferase (TPMT) enzyme activity involved in thiopurine metabolism; determining metabolite concentrations; gradual dose increase during empirical therapy; proper assessment of contraindications and monitoring of possible adverse reactions.