Allopurinol (Lopurin, Zyloprim) is used to treat gout, which is a form of arthritis caused by too much uric acid in your blood and joints. it is may also be used to lower your uric acid levels if you are receiving chemotherapy for cancer, or if you have certain types of kidney stones. Allopurinol works by reducing the production of uric acid in your body. If uric acid levels get too high, crystals can form in your joints and cause inflammation and pain (gout flares) or stones can form in your kidneys.