Allopurinol is an analogue of hypoxanthine, a breakdown product of adenine, which itself comes from adenosine, and is converted to alloxanthine by xanthine oxidase. The pharmacological actions of allopurinol are mainly due to alloxanthine. Allopurinol is a non-competitive inhibitor of this oxidase and has a long half-life of 18 to 30 hrs. The metabolites of xanthine oxidase are the relatively insoluble urates and uric acid. The deposition of urates in joints particularly the toe joints leading to the extremely painful development of gout. Allopurinol is used in the prophylaxis and long-term treatment of gout and hyperuricemia.