Xanthine Oxidase Inhibitor — Urate-lowering Therapy Pregnancy: There is inadequate evidence of the safety of allopurinol in human pregnancy, although it 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; allopurinol during breastfeeding is not recommended.

Allopurinol

Brand names: Zyloric

Used in: Gout

Allopurinol is a urate-lowering medicine used for the long-term prevention of gout and to reduce uric acid in some other settings. It is not used to treat an acute attack.

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 a low dose, e.g. 100 mg/day, and increase 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: Daily

Source: eMC SPC for Allopurinol 100 mg Tablet, §4.2. If dosage on a mg/kg bodyweight basis is required, 2 to 10 mg/kg bodyweight/day should be used. Extra caution if renal function is poor. Older people: in the absence of specific data, use the lowest dosage that produces satisfactory urate reduction. 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): correct existing hyperuricaemia and/or hyperuricosuria before starting cytotoxic therapy, ensure adequate hydration to maintain optimum diuresis, attempt alkalinisation of urine, and dose at the LOWER end of the recommended schedule. Monitoring: adjust dose by monitoring serum urate concentrations and urinary urate/uric acid levels at appropriate intervals. The §4.2 'Method of administration' sentence was truncated in the fetched source ('Allopurinol may be taken …') — confirm administration advice (e.g. relation to food) against the full SPC. No absolute adult maximum dose was stated in the fetched §4.2. eMC §4.5 was NOT captured in the fetched source, so the interactions listed below are taken from the US prescribing information (DailyMed, label date 2024-07-08) — verify against UK SPC §4.5.

Paediatric dose

Route: Oral

Frequency: Daily

Max: 400 mg daily

eMC §4.2 verbatim: 'Children under 15 years: 10 to 20 mg/kg bodyweight/day up to a maximum of 400 mg daily.' Expressed as a RANGE in the source, so no single dosePerKg value is recorded. Use in children is rarely indicated, except in malignant conditions (especially leukaemia) and certain enzyme disorders such as Lesch-Nyhan syndrome. Verify all under-18 dosing against a children's formulary before prescribing.

Dose adjustments

Renal

Impaired renal function may lead to retention of allopurinol 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 single doses of 100 mg at intervals longer than one day. If plasma oxipurinol monitoring is available, 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.

US labelling (FDA)

Reference — US labelling, may differ from UK

Gout : Prior to initiating treatment assess serum uric acid level, complete blood count, chemistry panel, liver and kidney function tests. Prophylactic treatment for gout flares is recommended. ( 2.1 , 2.2 ) Patients with normal kidney function: Initial dosage is 100 mg orally daily. Increase by 100 mg weekly increments until serum uric acid of 6 mg/dl or less is reached (maximum 800 mg daily). ( 2.3 ) Patients with impaired kidney function: The initial dosage is 50 mg orally daily. Follow recommendations for titration in patients with renal impairment until target serum uric acid level is reached. ( 2.6 ) See complete information in the Full Prescribing Information (FPI). Hyperuricemia …

Source: US FDA prescribing information (openFDA / DailyMed), label dated 2024-07-08. Accessed 2026-06-12. US dosing and indications can differ from UK practice — use UK sources for prescribing decisions.

Contraindications

Known hypersensitivity to allopurinol or to any of the components of the formulation (excipients listed in SPC §6.1)

Side effects

Rash (common); rare Stevens-Johnson syndrome / toxic epidermal necrolysis; very rare angioedema, drug eruption, alopecia
Hypersensitivity (uncommon), including delayed multi-organ hypersensitivity (hypersensitivity syndrome / DRESS); very rare anaphylactic reaction
Nausea, vomiting and diarrhoea (uncommon)
Liver function test abnormal (uncommon); hepatitis including hepatic necrosis and granulomatous hepatitis (rare)
Blood thyroid stimulating hormone increased (common)
Very rare agranulocytosis, aplastic anaemia and thrombocytopenia, particularly in individuals with impaired renal and/or hepatic function

Interactions

Bendamustine, thiazide diuretics, ampicillin and amoxicillin — may increase the risk of skin rash, which may be severe; renal impairment further increases the risk with thiazide diuretics (US label §7.1)
Capecitabine — avoid concomitant use (US label §7.2)
Mercaptopurine or azathioprine — reduce the mercaptopurine or azathioprine dose as recommended in the respective prescribing information (US label §7.2)
Pegloticase — discontinue allopurinol and refrain from initiating treatment (US label §7.2)
Interactions above are from the US prescribing information because eMC §4.5 was not captured in the fetched source — the UK SPC §4.5 list must be checked and this list replaced or extended

Clinical monograph

How it works

It inhibits xanthine oxidase, reducing production of uric acid.

Prescribing in practice

Start at a low dose and titrate to a target serum urate; provide anti-inflammatory cover (e.g. colchicine or an NSAID) during initiation, as starting can trigger a flare.
Do not stop it during an acute attack already established on treatment.
Rare but serious hypersensitivity can occur (higher risk in certain HLA-B*5801 backgrounds and in renal impairment); it interacts with azathioprine and mercaptopurine, whose doses must be markedly reduced.

Monitoring

Monitor serum urate and titrate to target; check renal function and remain alert to early rash or hypersensitivity.

Counselling the patient

Keep taking it long-term to prevent attacks, and do not stop it during a flare.
Report any rash promptly.
Drink plenty of fluids.

Evidence & guidelines

First-line urate-lowering therapy for gout, titrated to a urate target, per NICE NG219.

Reference: NICE NG219 (Gout); BSR Gout Guidelines 2017; 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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