A patient receiving allopurinol requires dose reduction of:

Allopurinol is an inhibitor of uric acid synthesis. It acts by inhibiting the enzyme 'xanthine oxidase. This drug is commonly prescribed to a patient on anticancer therapy because destruction of cells results in increased formation of uric acid. 6-Mercaptopurine is also metabolized by xanthine oxidase. If allopurinol is administered to a patient receiving 6-MP, toxicity may occur due to increased plasma concentration of 6-MP. Therefore if both are given concurrently, dose of 6-MP should be decreased to 25% of the normal.