Allopurinol is a drug used in the treatment of elevated uric acid levels, hyperuricemia, and its complications, such as gout. It belongs to a class of drugs known as xanthine oxidase inhibitors, meaning allopurinol inhibits an enzyme responsible for the production of uric acid. Uric acid builds up in patients to manifest itself as gout. Common side-effects include rash, joint stiffness, and toe pain. Allopurinol is used to treat gout and kidney stones. It may also be used in preventing or reducing the production of uric acid. It mostly used in patients that are receiving cancer treatment or chemotherapy. Allopurinol works by preventing an increase of uric acid in the body. Uric acid is the main cause of gouts and kidney problems. Allopurinol is available in the form of a solution, and it should be administered by the oral route with or without food. If you have a serious allergic reaction to this drug, then you should avoid using it. If you find skin rashes, stop taking Allopurinol immediately and seek medical attention immediately. Diarrhea and nausea are common side effects of Allopurinol that should not worry you unless they persist.