This study is a randomized controlled study conducted at five tertiary university hospitals. Patients who are 20-80 years old, diagnosed as having Inflammatory Bowel Disease(IBD) and who are planned to start thiopurines for the first time for the treatment of IBD are enrolled. Patients are assigned to the genotyping group or to the non-genotyping group. The patients who carry any heterozygotic variant among the three genes receive 50 mg azathioprine (AZA) or 25 mg of 6-mercaptopurine, while those who have any homozygotic variant are recommended to take other alternative drugs. The patients who do not carry any genetic variant or are assigned in non-genotyping group receive the standard dose of thiopurines based on the conventional approach.

Patients in the non-genotyping group receive the standard dose of thiopurines based on the conventional approach.

Inclusion Criteria:

• 20-80 years old
• Patients who were diagnosed as having IBD based on clinical, endoscopic, radiographic, and histological assessments,
• Patients who were planned to start thiopurines for the first time for the treatment of IBD.

Exclusion Criteria:

• Patients who had previous use of thiopurine
• Those who had abnormal laboratory findings prior to screening, including white blood cell (WBC) count < 3,000/μL, platelet (PLT) count < 100/μL, or elevation of aminotransferase more than twice the upper normal limits
• Those who were diagnosed other infectious diseases at the time of screening or receiving antibiotics within the previous 7 days;
• Those who were pregnant or lactating.

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