Xanthine Oxidase Inhibitor — Uric Acid Stone Prevention Pregnancy: Inadequate evidence of safety in human pregnancy; use only when there is no safer alternative and when the disease itself carries risk for the mother or unborn child. Breast-feeding not recommended.

Allopurinol

Brand names: Zyloric

Allopurinol is a xanthine oxidase inhibitor used to lower urate in the long-term prevention of gout and in the management of uric acid and calcium oxalate renal stones and tumour lysis-related hyperuricaemia.

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, increased only if serum urate response is unsatisfactory

Route: oral

Frequency: once daily

Max: 900 mg daily (severe conditions)

Suggested schedules: 100-200 mg daily in mild conditions; 300-600 mg daily in moderately severe conditions; 700-900 mg daily in severe conditions. If mg/kg basis required, 2-10 mg/kg bodyweight/day. Older people: use lowest dosage producing satisfactory urate reduction. High urate turnover conditions (e.g. neoplasia, Lesch-Nyhan): dose at lower end of schedule; correct hyperuricaemia before cytotoxic therapy with adequate hydration and urine alkalinisation. Adjust dose by monitoring serum urate and urinary urate/uric acid levels.

Paediatric dose

Dose: 10 mg/kg

Route: oral

Frequency: daily (total daily dose range 10-20 mg/kg/day)

Max: 400 mg daily

Children under 15 years: 10 to 20 mg/kg bodyweight/day up to a maximum of 400 mg daily. Use in children is rarely indicated, except in malignant conditions (especially leukaemia) and certain enzyme disorders such as Lesch-Nyhan syndrome.

Dose adjustments

Renal

Impaired renal function may lead to drug/metabolite retention. In severe renal insufficiency, use less than 100 mg per day or single 100 mg doses at longer intervals than one day; if oxipurinol levels can be monitored, adjust to keep plasma oxipurinol below 100 micromol/litre. Removed by renal dialysis; if dialysis 2-3 times/week, consider 300-400 mg immediately after each dialysis with none in 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.

Paediatric weight-based calculator

Patient weight (kg)

Verify in a children's formulary

Contraindications

Known hypersensitivity to allopurinol or to any of the components of the formulation

Side effects

Rash (common)
Liver function test abnormal (uncommon)
Hypersensitivity (uncommon)
Vomiting, nausea, diarrhoea (uncommon)
Stevens-Johnson syndrome/toxic epidermal necrolysis (rare)

Interactions

Mercaptopurine or azathioprine (reduce their dose)
Bendamustine, thiazide diuretics, ampicillin and amoxicillin (increased risk of skin rash)
Capecitabine (avoid concomitant use)
Pegloticase (discontinue allopurinol and refrain from initiating)

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 and urinary urate.

Prescribing in practice

It can rarely cause severe, life-threatening hypersensitivity including DRESS and Stevens-Johnson syndrome, so start at a low dose, warn the patient, and stop immediately and permanently at the first sign of rash; risk is higher in renal impairment and HLA-B*5801 carriers.
Starting therapy can precipitate an acute attack, so introduce it with gout prophylaxis cover and not during an acute flare.
It markedly increases the toxicity of azathioprine and mercaptopurine and interacts with ciclosporin; reduce the dose in renal impairment.

Monitoring

Monitor serum urate to guide titration to target and check renal function periodically, especially in renal impairment.

Counselling the patient

Stop the tablets and seek urgent medical advice if you develop a rash.
Keep taking it regularly even when free of attacks, and continue it during a flare once established.
Drink plenty of fluids.

Evidence & guidelines

Allopurinol is the first-line urate-lowering therapy with efficacy and safety established in the SPC and NICE and British gout guidance.

Reference: EAU Urolithiasis Guidelines 2024; NICE CG177 (Gout); 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.

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