It's a xanthine oxidase inhibitor. It reduces uric acid levels and keeps urate in the blood at the correct level.
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It’s used to treat gout.
The usual starting dose is between 100 mg and 300 mg daily, which may be increased if the dose is ineffective.
Side-effects are uncommon, but the most common are skin rashes, drowsiness or dizziness, nausea and vomiting. Allopurinol can also cause an increase in gout symptoms in the first few months.