Allopurinol

What it is

Allopurinol is a xanthine oxidase inhibitor: it blocks the enzyme that makes uric acid, so less is produced and blood levels fall over weeks. Lower uric acid gradually dissolves the crystal deposits in joints that cause gout attacks. It is a long-term preventive treatment rather than something taken for a flare in progress.

Used for

• Gout

What to know while taking it

It is taken with food, and generous fluid intake helps the kidneys clear the uric acid that is being shifted. A gout flare in the early weeks is expected and is not a sign the drug is failing. Uric acid is followed to see whether the target is reached, creatinine and eGFR because kidney function governs how much is appropriate, and ALT for liver effects. It raises the effect of warfarin, so anticoagulation needs closer watching, and it is genuinely dangerous with azathioprine, whose breakdown depends on the very enzyme allopurinol blocks.

When to see a doctor

Any rash is a reason to stop and call a doctor the same day, because rare but severe skin reactions to allopurinol begin that way, especially if accompanied by fever, mouth ulcers or peeling.

Worth knowing

• Any rash means stop and call a doctor
• Drink enough water
• A gout flare at the start is expected

Markers to watch

• Uric acid
• Creatinine
• eGFR
• ALT