<p>Allopurinol is a xanthine oxidase inhibitor which is administered orally. It 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 analogue of the natural purine base, hypoxanthine. It is an inhibitor of xanthine oxidase, the enzyme responsible for the conversion of hypoxanthine to xanthine and xanthine to uric acid, the end product of purine metabolism. Allopurinol is approximately 90% absorbed from the GI tract. Peak plasma levels generally occur at 1.5 hours to 4.5 hours. It has a plasma half life of about 1 to 2 hours. Approximately 20% of the ingested Allopurinol is excreted in the faeces.</p>