Steroids have an important role in the management of patients with inflammatory bowel disease (IBD). Generally, systemic corticosteroids are recommended for the induction of clinical response and remission for patients with moderate to severe IBD. However, a long-term use of steroids does not prevent disease relapse and only causes multiple adverse effects. Therefore, steroids should not be used a maintenance therapy and steroid-free remission should be targeted for patients with IBD. Thiopurines are non-specific immunosuppressive agents and are well known to be superior to placebo in terms of maintaining remission in patients with IBD. Therefore, thiopurines have been used for a maintenance treatment in IBD patients and for patients with steroid-dependency. Moreover, early thiopurine therapy has been reported to be associated with a reduced risk of intestinal resection in patients with Crohn’s disease. However, thiopurines could cause multiple adverse effects including myelotoxicity, which can be potentially life-threatening. Although genetic variations in thiopurine methyltransferase (TPMT) gene were less frequent among East Asians compared with Caucasians, the risks of leukopenia were generally higher than those reported form Caucasians. Recently, nudix hydrolase 15 (NUDT15) genetic variants were revealed to be strongly associated with the risk of leukopenia in East Asians and those observations were validated in other ethnic groups. Therefore, genotyping of NUDT15 gene before commencing thiopurines is recommended for preventing the development of significant myelosuppression.