Thiopurine S-methyltransferase (TPMT) catalyzes the methylation of thiopurine drugs, such as azathioprine and mercaptopurine, which are used in a variety of diseases. Several mutations in the TPMT gene correlate with low enzyme activity and subsequent adverse effects, mainly myelotoxicity. Hence, genotyping TPMT makes it possible to identify patients at high risk of drug toxicity and adjust dosage accordingly. However, further research about the availability of a reliable and universal screening method and more costeffectiveness studies are necessary.