Allopurinol is a xanthine oxidase inhibitor used to decrease high blood uric acid levels. Allopurinol is specifically used to prevent gout, prevent specific types of kidney stones, and for the high uric acid levels that can occur with chemotherapy. Allopurinol acts on purine catabolism, without disrupting the biosynthesis of purines. It reduces the production of uric acid by inhibiting the biochemical reactions immediately preceding its formation. Allopurinol is a structural analog of the natural purine base, hypoxanthine. It is an inhibitor of xanthine oxidase, the enzyme responsible for the conversion of hypoxanthine to xanthine and of xanthine to uric acid, the end product of purine metabolism in man. Allopurinol is metabolized to the corresponding xanthine analog, oxypurinol (Allopurinol), which also is an inhibitor of xanthine oxidase. Allopurinol is taken by mouth or injected into a vein. Common side effects, when used by mouth, include itchiness and rash. Common side effects when used by injection include vomiting and kidney problems.

Oxypurinol is an active metabolite of allopurinol and is an inhibitor of xanthine oxidase. Cardiome Pharma developed oxipurinol for the treatment of allopurinol-intolerant hyperuricemia (gout) and for the treatment of congestive heart failure. It is known, that inhibition of xanthine oxidase can improve myocardial work efficiency by sensitizing cardiac muscle cells to calcium ions, which are a key determinant of cardiac muscle function. However, all these studied were discontinued.