Allopurinol is a classified as a xanthine oxidase inhibitor. This means that it stops the enzyme xanthine oxidase from functioning correctly. Xanthine oxidase converts oxypurines (hypoxanthine and xanthine) to uric acid. It is found in many organs such as the liver, stomach, heart, brain, kidneys, and blood plasma. Xanthine oxidase converts hypoxanthine to xanthine and then xanthine to uric acid. Uric acid is a product of broken down foods and cells that are excreted by the kidneys. High levels of hypoxanthine and xanthine being converted to uric acid by xanthine oxidase or decreased kidney function can increase the level of uric acid in the blood. Too much uric acid in the blood builds up around joints and causes the pain and swelling associated with gout. An increase in uric acid is also associated with organ damage and failure.