1 INTRODUCTION Thiopurines are a class of immunosuppressant and antineoplastic agents that include mercaptopurine, azathioprine (prodrug of mercaptopurine) and thioguanine. They are widely used in treating inflammatory bowel disease, haematological malignancies, autoimmune diseases and various dermatological conditions.1 However, thiopurines are associated with significant adverse drug reactions, and 15–40% of patients will discontinue treatment due to intolerance.