Xanthine-oxidase inhibitors are used to treat gout, kidney stones, and lower uric acid levels during certain cancer treatments. These medications work by blocking the enzyme xanthine oxidase, which helps to reduce the production of uric acid in your body. Lowering uric acid levels can help prevent gout attacks and the formation of kidney stones. Xanthine-oxidase inhibitors are available in oral forms. Examples include allopurinol (Zyloprim) and febuxostat (Uloric).