Known brand names include Zyloprim.
A pill that lowers uric acid to prevent gout attacks, high uric acid during cancer treatment, and some repeat kidney stones. It blocks the enzyme that makes uric acid, and any rash is a reason to stop it, because rare skin reactions can be fatal.
Allopurinol blocks xanthine oxidase, the enzyme that turns purines, natural substances in the body, into uric acid, the final waste product of their breakdown. The body changes it into oxypurinol, which blocks the same enzyme and is thought to do most of the work. With the enzyme blocked, less uric acid is made, so levels fall in both the blood and the urine. Some of the purines that would have become uric acid are reused instead, and more of them leave in the urine.
It is well absorbed, and oxypurinol stays in the body far longer than allopurinol itself, with a half-life of about 15 hours. Control of gout attacks can take several months, and attacks may increase at first as uric acid deposits in the tissues break loose. The kidneys clear oxypurinol, so weak kidneys let it build up and call for lower doses. Its most serious risk is a severe allergic reaction with rash and organ damage, which is more likely in people who carry a gene variant called HLA-B*58:01.
Allopurinol treats gout, a type of arthritis in which uric acid collects in the joints and sets off sudden bouts of pain, swelling, redness, and heat. It also keeps uric acid from rising in people whose cancer treatment breaks down tumor cells, and it treats calcium oxalate kidney stones that keep coming back in people who pass too much uric acid in their urine. Over time, lower uric acid means fewer gout attacks and less risk of tophi (lumps of uric acid under the skin), joint damage, and stones. It prevents gout attacks but does not treat an attack once it has started.
Adults take it for gout and for kidney stones, while both adults and children take it during cancer treatment. It is a tablet taken by mouth at home, usually once or twice a day after a meal, along with plenty of fluids. Gout attacks can become more frequent in the first months, so a second medicine such as colchicine is often taken with it at the start. It is not recommended for high uric acid that causes no symptoms, and it has not been shown to be safe and effective in children with gout or kidney stones.
Rash is the most frequent side effect, and nausea, diarrhea, drowsiness, and higher liver test results also occur. Gout attacks often increase during the first months of treatment. The label treats severe skin and allergic reactions, including blistering and peeling of the skin and a whole-body reaction with fever and organ damage, as serious and sometimes fatal, along with liver injury, kidney problems, and low blood counts.