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Xanthine Oxidase Inhibitor — Urate-Lowering Therapy Pregnancy: Avoid in pregnancy; limited safety data

Allopurinol

Brand names: Zyloric

Adult dose

Dose: 100 mg once daily initially, increasing to 100-900 mg/day
Route: Oral
Frequency: Once daily (after food)
Max: 900 mg/day
Target serum urate under 360 micromol/L (under 300 micromol/L for tophaceous gout). Titrate up every 4 weeks

Paediatric dose

Dose: Seek specialist opinion N/A/kg
Route: Oral
Frequency: Seek specialist opinion
Max: Seek specialist opinion
Seek specialist opinion

Dose adjustments

Renal

Critical: eGFR 30-59: max 100-200 mg/day; eGFR 10-29: 100 mg on alternate days; eGFR under 10: 100 mg at longer intervals — dose carefully, risk of DRESS/SJS at high doses in renal impairment

Hepatic

Reduce dose in hepatic impairment

Paediatric weight-based calculator

Seek specialist opinion

Clinical pearls

  • Never start during an acute gout attack — wait until attack fully resolved; initiate colchicine or NSAID prophylaxis to cover first 6 months of ULT
  • HLA-B*5801 screening recommended in Han Chinese, Korean, and Thai patients before starting — high risk of severe SJS/DRESS in these populations
  • Azathioprine interaction is dangerous and potentially fatal — check all medications before prescribing; if combination unavoidable, reduce azathioprine dose by 75% and monitor FBC closely
  • Renal dosing in elderly is critical — most elderly patients have eGFR under 60; start at 50-100 mg and titrate slowly
  • NICE CG177: Allopurinol is first-line ULT; check serum urate every 4 weeks during titration, then annually

Contraindications

  • Do not start during acute gout attack (will prolong attack)
  • Hypersensitivity to allopurinol — HLA-B*5801 genotype strongly associated with severe cutaneous reactions

Side effects

  • Rash (stop immediately — can progress to SJS or DRESS)
  • GI upset
  • Acute gout flare on initiation
  • Hepatotoxicity (rare)
  • DRESS syndrome (Drug Reaction with Eosinophilia and Systemic Symptoms)

Interactions

  • Azathioprine / mercaptopurine (allopurinol inhibits xanthine oxidase — dramatically increases toxicity; reduce azathioprine dose by 75%)
  • Warfarin (enhanced anticoagulant effect)
  • Ampicillin / amoxicillin (increased rash rate)
  • ACE inhibitors (increased risk of hypersensitivity reactions)

Monitoring

  • Serum urate (target under 360 micromol/L)
  • eGFR and renal function
  • LFTs
  • FBC (if on azathioprine)
  • Skin (rash — stop immediately)

Reference: BNFc; BNF 90; NICE CG177 (Gout); BSR Gout Guidelines 2017; MHRA HLA-B*5801 guidance. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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