For malignancy: initiate therapy with drastically reduced starting doses. Reduce starting dose by 10-fold and reduce frequency to thrice weekly instead of daily (e.g. 10 mg/m2/day given 3 days/week). During therapy, adjust mercaptopurine doses based on the degree of myelosuppression and disease-specific guidelines. It usually takes at least 4-6 weeks of stable dosing to reach steady state after each dose adjustment. If myelosuppression occurs, emphasis should be on reducing mercaptopurine over other agents. For nonmalignancy: consider alternative nonthiopurine immunosuppressant therapy.