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Pharmacology · Xanthine oxidase inhibitors

Allopurinol !

The classic tablet that lowers uric acid in gout and in cancer treatment. A rash is a stop sign: rare but severe skin reactions occur.

Updated 2026-11-01

The figure shows cow's-milk xanthine dehydrogenase with oxipurinol, the active metabolite of allopurinol, bound at the molybdenum centre; the human enzyme has the same site.

Target in 3D: Xanthine oxidase. PDB structure 3BDJ (RCSB PDB, public domain). View the structure at RCSB PDB

Mechanism of action

Allopurinol is a structural analogue of hypoxanthine. It and its metabolite oxipurinol inhibit xanthine oxidase, the enzyme that makes uric acid from hypoxanthine and xanthine. Less uric acid is formed, and the more soluble precursors are reused for nucleotide synthesis or excreted. Lower urate lets crystals dissolve over months and reduces tophi, flares and uric acid stones.

  1. Hypoxanthine analogue, oxidised to oxipurinol
  2. Oxipurinol blocks xanthine oxidase
  3. Less uric acid is produced
  4. Urate crystals slowly dissolve

Pharmacokinetics

Absorption
Well absorbed by mouth (about 90%)
Metabolism
Quickly oxidised to oxipurinol, the active metabolite that carries most of the effect
Elimination
Oxipurinol is cleared by the kidneys, so it builds up in kidney impairment and the starting dose is lower

Indications

Adverse effects

Common

  • Rash
  • Gout flares when starting
  • Nausea and diarrhoea
  • Raised liver enzymes

Contraindications

  • Previous hypersensitivity reaction to allopurinol

Key interactions

Azathioprine and mercaptopurine
Allopurinol blocks their inactivation by xanthine oxidase; exposure rises and marrow suppression can be severe, so their dose is reduced.
Ampicillin and amoxicillin; thiazide diuretics; bendamustine
Higher risk of serious skin reactions.
Coumarin anticoagulants (dicumarol)
Its half-life lengthens; the prothrombin time is rechecked.
Chlorpropamide
Competes for renal excretion; risk of hypoglycaemia in kidney impairment.

What is monitored

  • Serum uric acid, blood count, chemistry, liver and kidney tests before starting and during treatment
  • HLA-B*58:01 test considered before starting in people from populations where the allele is common
  • Skin: any rash during the first months

Administration and safety

  • Oral tablet.
  • It is started low and raised slowly, especially in kidney impairment.
  • Flare prophylaxis (colchicine or an anti-inflammatory) usually accompanies the first months.

Nursing care

  • Before the first dose, ask about azathioprine or mercaptopurine and check the kidney function result.
  • Teach to stop it and seek care at the first rash, blistering, mouth or eye sores, painful urination or blood in the urine.
  • Explain that flares can increase at first and that the flare medicine is continued.
  • Check that the HLA-B*58:01 result is available when it was requested.

Pregnancy and breastfeeding

It may cause fetal harm; the treating team weighs it case by case.

Test yourself

Think of the answer, then tap the question to see it.

Which genetic marker predicts severe skin reactions to allopurinol?

HLA-B*58:01, more frequent in some Asian (Han Chinese, Korean, Thai), African and Native Hawaiian populations.

A transplant patient on azathioprine starts allopurinol. What is the danger?

Azathioprine's metabolite 6-mercaptopurine is inactivated by xanthine oxidase; allopurinol blocks it, exposure rises and severe myelosuppression can follow unless azathioprine is reduced.

Why is allopurinol given before chemotherapy for some leukaemias and lymphomas?

To prevent the rise in uric acid when many tumour cells die (tumour lysis), which can cause uric acid nephropathy.

Is allopurinol used to treat an acute gout attack?

No. It is long-term urate-lowering therapy; flares are treated with colchicine, NSAIDs or glucocorticoids.

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: Allopurinol: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2023); DailyMed - ALLOPURINOL tablet (DailyMed (U.S. National Library of Medicine), 2023); Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 42 (McGraw Hill, 2023); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026).

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Representative model, for educational purposes only.