Allopurinol and its metabolite, oxipurinol (alloxanthine), decrease the production of uric acid by inhibiting the action of xanthine oxidase, the enzyme that converts hypoxanthine to xanthine and xanthine to uric acid. Also, allopurinol increases reutilization of hypoxanthine and xanthine for nucleotide and nucleic acid synthesis via an action involving the enzyme hypoxanthine - guanine phosphoribosyltransferase (HGPRTase). The resultant increase in nucleotide concentration leads to feedback inhibition of de novo purine synthesis.
Allopurinol thereby decreases uric acid concentrations in both serum and urine.

Risk benefit should be considered when the foIlowing medical problems exist: renal function impairment or any illness that may predispose to a change in renal function, such as: congestive heart disease, diabetes meIlitus, hypertension and sensitivity to allopurinol.

Although adequate and well - controIled studies in humans have not been done, 3 reports indicate no evidence of birth defects in offspring of women receiving allopurinol during pregnancy.
Allopurinol is distributed into breast milk. Whether this toxic medication may cause adverse effects in the nursing infant has not been determined. However, problems in humans have not been documented.

Combinations containing any of the foIlowing medications, depending on the amount present, may also interact with this medication:
Acidifiers, urinary, such as: Ammonium chloride, Ascorbic acid, Potassium or sodium phosphate - Alcohol - Diazoxide - pyrazinamide - Mecamylamine - Amoxicillin - Ampicillin - Anticoagulants, coumarin Antineoplastics Azathioprine - Mercaptopurine - Chlorpropamide - Dacarbazine- Diuretics, thiazide- Probenecid- Sulfinpyrazone - Vidarabine, systemic - Xanthines, such as: Aminophylline, oxtriphylline, Theophylline.

Those indicating need for medical attention; Incidence more frequent: Dermatitis, allergic. Incidence rare: Agranulocytosis; anemia; angiitis (vasculitis); hypersensitivity; aplastic anemia; dermatitis, exfoliative; erythema multiforme; hepatotoxicity; hypersensitivity reaction, allopurinol - induced; loosening of fingernails; necrolysis; toxic epidermal; neuritis, peripheral; renal calculus, xanthine; renal failure, acute; stevens - johnson syndrome; thrombocytopenia; unexplained nosebleeds.

Usual adult and adolescent dose: Initial - Oral, 100 mg once a day, to be increased by 100 mg per day at one - week intervals until the desired serum uric acid concentration is attained, not to exceed the maximum recommended dosage of 800 mg per day.
Maintenance - Oral, 100 to200 mg 2 or 3 times a day; or 300 mg as a single dose once a day. The usual maintenance dose is 200 to 300 mg per day in mild gout or 400 to 600 mg per day in moderately severe tophaceous gout.
Neoplastic disease therapy: Initial -Oral, 600 to 800 mg per day starting 12 hours to 3 days (preferably 2 to 3 days) prior to initiation of chemotherapy or radiation therapy.
Maintenance - Dosage should be based on serum uric acid determinations performed approximately forty - eight hours after initiation of allopurinol therapy and periodically thereafter.
Allopurinol should be discontinued following the period of tumor regression.
Antiurolithic (uric acid calculi) -Oral, 100 to 200 mg 1 to 4 times a day, or 300 mg as a single dose once a day. Antiurolithic (calcium oxalate calculi) - Oral, 200 to 300 mg a day as a single does or in divided doses. Usual adult prescribing limits: 300 mg per dose; 800 mg per day.
Usual pediatric dose: Antihyperuricemic, in neoplastic disease therapy - Children up to 6 years of age: Oral, 50 mg 3 times a day.
Children 6 to 10 years of age: Oral, 100mg 3 times a day; or 300 mg as a single dose once a day.