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Xanthine Oxidase Inhibitor — Urate-lowering Therapy Pregnancy: Avoid — insufficient data; animal studies suggest teratogenicity. Acute gout in pregnancy: manage with colchicine or low-dose prednisolone.

Allopurinol

Brand names: Zyloric

Adult dose

Dose: Gout (urate-lowering): 100mg OD initially (start during quiescent period — NOT during acute flare); increase by 100mg every 4 weeks to target serum urate <360 micromol/L (ideally <300 micromol/L in tophaceous gout); usual maintenance 200–300mg OD; maximum 900mg daily. Tumour lysis syndrome prophylaxis: 300mg OD starting 24h before chemotherapy.
Route: Oral
Frequency: Once daily (after meals)
Max: 900mg daily
DO NOT start during acute gout flare — can prolong or worsen the acute attack by mobilising urate crystals. Start prophylactic colchicine 500 micrograms BD (or low-dose NSAID) for 3–6 months when starting allopurinol to prevent flares. Dose titrated against serum urate target (not symptoms). Continue lifelong in most patients.

Paediatric dose

Dose: 5 mg/kg
Route: Oral
Frequency: Once daily
Max: 400mg daily
BNFc: Enzyme disorder associated with hyperuricaemia (Lesch-Nyhan): 10–20mg/kg/day. Tumour lysis syndrome prophylaxis: 10mg/kg/day (max 400mg daily). Seek specialist paediatric opinion.

Dose adjustments

Renal

eGFR 10–50: start at 50mg OD; increase slowly. eGFR <10: 50–100mg every 2–3 days. Dose titrated to serum urate target regardless of eGFR — go slowly.

Hepatic

Use with caution in hepatic impairment — hepatic metabolism; monitor LFTs.

Paediatric weight-based calculator

BNFc: Enzyme disorder associated with hyperuricaemia (Lesch-Nyhan): 10–20mg/kg/day. Tumour lysis syndrome prophylaxis: 10mg/kg/day (max 400mg daily). Seek specialist paediatric opinion.

Clinical pearls

  • Azathioprine interaction is potentially fatal — ALWAYS check if patient is on azathioprine (or mercaptopurine) before prescribing allopurinol. If both needed, reduce azathioprine dose by 75% and monitor FBC closely. Many hospitals have alerts for this combination
  • HLA-B*5801 screening: mandatory before prescribing in Han Chinese, Thai, and Korean patients — this allele confers very high risk of SJS/TEN (up to 100-fold increased risk in renal impairment)
  • Colchicine prophylaxis: start 500 micrograms BD at the same time as allopurinol — reduces initiation flares by 85%. Continue for minimum 3–6 months (or until serum urate stable for 6 months)
  • Serum urate target: <360 micromol/L (6mg/dL) for most; <300 micromol/L (5mg/dL) for tophaceous gout — check urate every 4 weeks during titration

Contraindications

  • Acute gout flare (do not initiate — wait until flare resolves)
  • Hypersensitivity to allopurinol
  • HLA-B*5801 allele (Han Chinese, Thai, Korean ancestry — high risk of SJS/TEN — screen before prescribing per MHRA/NICE)

Side effects

  • Rash (common — mild; if severe rash develops, stop immediately and do not rechallenge)
  • Stevens-Johnson syndrome / TEN (rare — particularly in HLA-B*5801 carriers and CKD)
  • Gout flare (paradoxical — at initiation; prevented by prophylactic colchicine)
  • Nausea, diarrhoea
  • Hepatotoxicity (rare)
  • DRESS syndrome (drug reaction with eosinophilia and systemic symptoms — rare)

Interactions

  • Azathioprine / mercaptopurine — CRITICAL: allopurinol inhibits xanthine oxidase, the enzyme that metabolises azathioprine; levels rise 3–4 fold; life-threatening bone marrow suppression. Reduce azathioprine dose by 75% if combination unavoidable; normally avoid
  • Ampicillin / amoxicillin — increased incidence of rash (not a contraindication but warn patient)
  • Warfarin — allopurinol increases warfarin levels; monitor INR
  • Ciclosporin — increases ciclosporin levels; monitor

Monitoring

  • Serum urate (every 4 weeks during titration; then 6 monthly when stable — target <360 micromol/L)
  • FBC and LFTs (baseline and 3 monthly in first year)
  • Renal function
  • Gout flare frequency

Reference: BNFc; BNF 90; NICE NG219 (Gout); BSR Gout Guidelines 2017. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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