Allopurinol (brand names include: Zyloprim / Allohexal / Allosig / Progout / Zyloric / Puricos) belongs to a group of medicines called anti-uricaemics.
These medicines work by reducing the formation and accumulation of uric acid in the body. Excess amounts of uric acid in the blood may lead to the development of crystals which deposit in the joints, causing pain, swelling and tenderness.
Allopurinol is used in the treatment of:
gout or gouty arthritis (a condition of painful swollen joints caused by uric acid crystals);
kidney stones (uric acid crystals in the kidney);
skin tophi (uric acid crystals beneath the skin);
other rare conditions where high levels of uric acid occur in the blood (eg. Lesch-Nyhan syndrome).
Your doctor may have prescribed Allopurinol for another reason.