Allopurinol is a medication used to decrease high blood uric acid levels. It is specifically used to prevent gout, prevent specific types of kidney stones and for the high uric acid levels that can occur with chemotherapy. It is taken by mouth or injected into a vein.Common side effects when used by mouth include itchiness and rash. Common side effects when used by injection include vomiting and kidney problems. While not recommended historically, starting allopurinol during an attack of gout appears to be safe. In those already on the medication, it should be continued even during an acute gout attack. While use during pregnancy does not appear to result in harm, this use has not been well studied. Allopurinol is in the xanthine oxidase inhibitor family of medications.Allopurinol was approved for medical use in the United States in 1966. It is on the World Health Organizations List of Essential Medicines. Allopurinol is available as a generic medication. In 2020, it was the 42nd most commonly prescribed medication in the United States, with more than 15 million prescriptions. Medical uses
Gout
Allopurinol is used to reduce urate formation in conditions where urate deposition has already occurred or is predictable.
Drug interactions
Drug interactions are extensive, and are as follows:
Azathioprine and 6-mercaptopurine: Azathioprine is metabolised to 6-mercaptopurine which in turn is inactivated by the action of xanthine oxidase - the target of allopurinol. Giving allopurinol with either of these drugs at their normal dose will lead to overdose of either drug;  only one-quarter of the usual dose of 6-mercaptopurine or azathioprine should be given;
Didanosine: plasma didanosine Cmax and AUC values were approximately doubled with concomitant allopurinol treatment; it should not be co-administered with allopurinol and if it must be, the dose of should be reduced and the person should be closely monitored.Allopurinol may also increase the activity or half-life of the following drugs, in order of seriousness and certainty of the interaction:
Ciclosporin
Coumarin anticoagulants, such as warfarin (reported rarely, but is serious when it occurs)
Vidarabine
Chlorpropamide
Phenytoin
Theophylline
Cyclophosphamide, doxorubicin, bleomycin, procarbazine, mechlorethamineCo-administration of the following drugs may make allopurinol less active or decrease its half-life:
Salicylates and medicines that increase the secretion of uric acid
furosemide (see more on diuretics below)Co-administration of the following drugs may cause hypersensitivity or skin rash:
Ampicillin and amoxicillin
Diuretics, in particular thiazides, especially in renal impairment
Angiotensin-converting-enzyme inhibitors (ACE inhibitors)

Pharmacology
A common misconception is that allopurinol is metabolized by its target, xanthine oxidase, but this action is principally carried out by aldehyde oxidase. The active metabolite of allopurinol is oxipurinol, which is also an inhibitor of xanthine oxidase. Allopurinol is almost completely metabolized to oxipurinol within two hours of oral administration, whereas oxipurinol is slowly excreted by the kidneys over 18–30 hours.