Xanthine oxidase inhibitor / Urate-lowering therapy Pregnancy: There is inadequate evidence of safety in human pregnancy, although allopurinol has been in wide use for many years without apparent ill consequence; use in pregnancy only when there is no safer alternative and when the disease itself carries risk for the mother or unborn child. Allopurinol and its metabolite oxipurinol are excreted in human breast milk and allopurinol during breast-feeding is not recommended.

Allopurinol

Brand names: Zyloric

Allopurinol is a xanthine oxidase inhibitor used as urate-lowering therapy to prevent recurrent gout and uric acid nephrolithiasis, and to manage hyperuricaemia including tumour lysis.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: Introduce at low dosage, e.g. 100 mg/day, increasing only if the serum urate response is unsatisfactory. Suggested schedules: 100 to 200 mg daily in mild conditions; 300 to 600 mg daily in moderately severe conditions; 700 to 900 mg daily in severe conditions

Route: Oral

Frequency: Once a day after a meal; if the daily dosage exceeds 300 mg and gastrointestinal intolerance is manifested, a divided-dose regimen may be appropriate

Source: UK SPC (eMC) for Allopurinol 100 mg Tablet, §4.2 (https://www.medicines.org.uk/emc/product/100235/smpc). Extra caution should be exercised if renal function is poor. If dosage on a mg/kg bodyweight basis is required, 2 to 10 mg/kg bodyweight/day should be used. OLDER PEOPLE: in the absence of specific data, the lowest dosage which produces satisfactory urate reduction should be used, with particular attention to the renal impairment advice. HEPATIC IMPAIRMENT: reduced doses should be used; periodic liver function tests are recommended during the early stages of therapy. HIGH URATE TURNOVER CONDITIONS (e.g. neoplasia, Lesch-Nyhan syndrome): it is advisable to correct existing hyperuricaemia and/or hyperuricosuria with allopurinol before starting cytotoxic therapy; ensure adequate hydration to maintain optimum diuresis and attempt alkalinisation of urine to increase solubility of urinary urate/uric acid; the allopurinol dosage should be at the lower end of the recommended schedule. MONITORING: adjust the dosage by monitoring serum urate concentrations and urinary urate/uric acid levels at appropriate intervals. §4.4 also advises considering HLA-B*5801 screening before starting treatment in patient subgroups where the prevalence of this allele is known to be high (e.g. Han Chinese, Thai, Korean descent), because of the risk of hypersensitivity syndrome and SJS/TEN. §4.5 was not retrieved in this bundle — the interaction entries below come from §4.4; verify the full §4.5. No absolute adult maximum daily dose is stated in the fetched §4.2 beyond the schedules above.

Paediatric dose

Route: Oral

Frequency: Daily (total daily dose)

Max: Up to a maximum of 400 mg daily

SPC §4.2 paediatric population: 'Children under 15 years: 10 to 20 mg/kg bodyweight/day up to a maximum of 400 mg daily.' A range (10 to 20 mg/kg/day) is given rather than a single figure, so dosePerKg is left null — use the quoted range. Use in children is rarely indicated, except in malignant conditions (especially leukaemia) and certain enzyme disorders such as Lesch-Nyhan syndrome. Verify against a children's formulary.

Dose adjustments

Renal

Allopurinol and its metabolites are excreted by the kidney, so impaired renal function may lead to retention of the drug and/or its metabolites with prolongation of plasma half-lives. In severe renal insufficiency it may be advisable to use less than 100 mg per day, or to use single doses of 100 mg at longer intervals than one day. If plasma oxipurinol concentrations can be monitored, adjust the dose to maintain plasma oxipurinol below 100 micromol/litre (15.2 mg/litre). Allopurinol and its metabolites are removed by renal dialysis; if dialysis is required two to three times a week, consider an alternative schedule of 300-400 mg allopurinol immediately after each dialysis with none in the interim.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Hypersensitivity to allopurinol or to any of the components of the formulation

Side effects

Common: rash
Common: blood thyroid stimulating hormone increased
Uncommon: hypersensitivity (including a delayed multi-organ hypersensitivity disorder / DRESS with fever, rashes, vasculitis, lymphadenopathy)
Uncommon: nausea, vomiting, diarrhoea; abnormal liver function tests
Rare: Stevens-Johnson syndrome / toxic epidermal necrolysis; hepatitis (including hepatic necrosis and granulomatous hepatitis)
Very rare: agranulocytosis, aplastic anaemia and thrombocytopenia — particularly in individuals with impaired renal and/or hepatic function

Interactions

Thiazide and other diuretics in patients with chronic renal impairment — may be at increased risk of hypersensitivity reactions including SJS/TEN associated with allopurinol; extra vigilance required (§4.4)
Diuretics or ACE inhibitors used to treat hypertension or cardiac insufficiency — patients may have concomitant impairment of renal function, so allopurinol should be used with care in this group (§4.4)
§4.5 was not retrieved in this bundle — verify the full interaction section (the SPC §4.2 cross-refers to it)

Clinical monograph

How it works

It and its active metabolite oxypurinol inhibit xanthine oxidase, reducing the conversion of hypoxanthine and xanthine to uric acid and thereby lowering serum urate.

Prescribing in practice

Stop immediately and seek advice if a rash develops, as allopurinol can cause severe hypersensitivity (including DRESS and Stevens-Johnson syndrome), with higher risk in those carrying HLA-B*58:01.
Do not start during an acute attack; begin under cover of an anti-inflammatory or colchicine prophylaxis to avoid precipitating a flare, and start low with slow titration in renal impairment.
It markedly potentiates azathioprine and mercaptopurine, requiring substantial dose reduction of those drugs to avoid life-threatening myelosuppression.

Monitoring

Monitor serum urate to a target, together with renal function, and review for any sign of hypersensitivity early in treatment.

Counselling the patient

Keep taking it every day, including during a gout flare, and do not stop just because an attack settles.
Report any skin rash promptly.
Maintain a good fluid intake while on treatment.

Evidence & guidelines

Urate-lowering therapy and an HLA-B*58:01-informed approach in at-risk groups are supported by NICE gout guidance and MHRA hypersensitivity advice.

Reference: NICE NG219 Gout; BSR Gout Guidelines; EULAR 2022; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways