Allopurinol is a xanthine oxidase inhibitor used to decrease high blood uric acid levels. Allopurinol is specifically used to prevent gout, prevent specific types of kidney stones, and for the high uric acid levels that can occur with chemotherapy. Allopurinol acts on purine catabolism, without disrupting the biosynthesis of purines. It reduces the production of uric acid by inhibiting the biochemical reactions immediately preceding its formation. Allopurinol is a structural analog of the natural purine base, hypoxanthine. It is an inhibitor of xanthine oxidase, the enzyme responsible for the conversion of hypoxanthine to xanthine and of xanthine to uric acid, the end product of purine metabolism in man. Allopurinol is metabolized to the corresponding xanthine analog, oxypurinol (Allopurinol), which also is an inhibitor of xanthine oxidase. Allopurinol 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.

Allopurinol riboside is a metabolite of allopurinol, a xanthine oxidase inhibitor indicated for the management of patients with leukemia, lymphoma, and solid tumor malignancies who are receiving cancer therapy which causes elevations of serum and urinary uric acid levels and who cannot tolerate oral therapy. Allopurinol riboside is not an inhibitor of xanthine oxidase. Allopurinol riboside is commonly thought to be directly synthesized by purine nucleoside phosphorylase (PNP) in vivo. Allopurinol riboside competitively inhibits the action of PNP on inosine in vitro. Allopurinol riboside potently inhibits growth in vitro of promastigotes of Leishmania species. Patients with American cutaneous leishmaniasis who received allopurinol riboside had clinical improvement. In addition, allopurinol riboside demonstrated some effectivity against Trypanosoma cruzi infections in animals.