Allopurinol

Generic Name: Allopurinol

Brand Names: Zyloprim, Aloprim

Allopurinol reduces uric acid production, preventing gout attacks and treating hyperuricemia.

RheumatologyGoutHyperuricemia

Drug Class

Xanthine Oxidase Inhibitor (Antihyperuricemic)

Pregnancy

Category C — Animal reproduction studies have shown some adverse fetal effects at high doses; there are no adequate well-controlled studies in pregnant women. Allopurinol crosses the placenta. Use during pregnancy only if the potential benefit clearly justifies the potential risk to the fetus.

Available Forms

Oral tablet (100 mg, 300 mg), IV powder for solution (500 mg single-dose vial — Aloprim)

What It's Used For

Dosage Quick Reference

These are general dosage guidelines. Your doctor will determine the appropriate dose for your specific situation.

Condition	Starting Dose	Maintenance Dose
Mild gout / hyperuricemia	100 mg orally once daily	Titrate by 100 mg every 2–5 weeks to serum urate < 6 mg/dL; usual range 200–400 mg/day
Moderate-to-severe tophaceous gout	100 mg once daily	400–600 mg/day; max 800 mg/day in divided doses
Recurrent calcium oxalate stones	200–300 mg/day	Adjust based on 24-hour urinary urate excretion
Tumor lysis syndrome prevention	600–800 mg/day for 2–3 days before chemotherapy	300–400 mg/day; reduce in renal impairment
Renal impairment (CrCl 10–20 mL/min)	100 mg every other day	Titrate cautiously; max 200 mg/day

Side Effects

Common Side Effects:

Rash
Acute gout flares (especially during initiation)
Nausea
Diarrhea
Elevated liver enzymes
Drowsiness
Headache

Serious Side Effects:

Severe cutaneous adverse reactions (SJS, TEN, DRESS)
Allopurinol hypersensitivity syndrome
Hepatotoxicity
Bone marrow suppression
Vasculitis
Renal failure

Drug Interactions

Allopurinol inhibits xanthine oxidase, which also metabolizes several other drugs, leading to clinically important interactions.

Azathioprine and 6-mercaptopurine: Allopurinol blocks the inactivation of these purine analogs, dramatically increasing their levels and the risk of severe bone marrow suppression. Reduce azathioprine or 6-MP dose by 65–75% if combination is unavoidable, and monitor CBC closely.
Warfarin: Allopurinol may inhibit warfarin metabolism, enhancing its anticoagulant effect. Monitor INR more frequently when starting, stopping, or changing the allopurinol dose.
Ampicillin / Amoxicillin: Co-administration increases the incidence of skin rash by approximately threefold. Consider alternative antibiotics when feasible.
Thiazide and loop diuretics: Diuretics raise serum urate and may also increase the risk of allopurinol hypersensitivity, particularly in patients with renal impairment. Monitor renal function and skin reactions closely.
ACE inhibitors (e.g., lisinopril, enalapril): Combination has been associated with an increased risk of hypersensitivity reactions and leukopenia, especially in patients with chronic kidney disease.
Theophylline: Allopurinol reduces theophylline clearance at higher allopurinol doses, raising the risk of theophylline toxicity. Monitor theophylline levels.

Additional Information

Allopurinol is a xanthine oxidase inhibitor used primarily for the management of gout and hyperuricemia. This medication works by reducing the body's production of uric acid, helping to prevent gout attacks and complications associated with elevated uric acid levels.

Mechanism of Action

Allopurinol and its active metabolite oxypurinol (also called alloxanthine) work by inhibiting xanthine oxidase, the enzyme responsible for the final two steps in uric acid synthesis: the conversion of hypoxanthine to xanthine and xanthine to uric acid. By blocking this enzyme, allopurinol reduces serum and urinary uric acid concentrations. Unlike uricosuric agents that increase uric acid excretion, allopurinol decreases uric acid production, making it effective regardless of renal function. The reduction in serum urate levels eventually leads to dissolution of existing urate crystal deposits, though this process takes months to years.

Available Formulations

Allopurinol is available as oral tablets in 100 mg and 300 mg strengths. Generic formulations are widely available and cost-effective. The tablets can be taken with food to minimize gastrointestinal upset. An intravenous formulation is available for patients unable to take oral medications, typically used in the prevention or treatment of tumor lysis syndrome.

Medical Uses

Allopurinol is indicated for the management of gout, including acute gouty arthritis prevention, tophi reduction, and prevention of urate nephropathy. It is also used for managing hyperuricemia secondary to cancer treatment (tumor lysis syndrome) and for recurrent calcium oxalate kidney stones with hyperuricosuria. Off-label uses include prevention of ischemia-reperfusion injury and treatment of certain metabolic disorders. Treatment goals in gout typically aim for serum urate levels below 6 mg/dL, or below 5 mg/dL in patients with tophi.

Dosing Guidelines

The initial recommended dose for gout is 100 mg daily, with gradual increases of 100 mg at weekly intervals until serum urate targets are achieved or the maximum dose of 800 mg daily is reached. Most patients achieve target levels with 300-600 mg daily. For patients with renal impairment, lower starting doses (50-100 mg) and slower titration are recommended. The daily dose may be taken once daily or divided into multiple doses. Doses greater than 300 mg should be divided. Adequate hydration is important to prevent xanthine stone formation during initial therapy.

Important Safety Information

Allopurinol can cause severe hypersensitivity reactions, including Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN), which can be fatal. The HLA-B*5801 allele is strongly associated with severe cutaneous adverse reactions, particularly in patients of Han Chinese, Thai, and Korean ancestry. Screening for this allele is recommended before initiating therapy in high-risk populations. Allopurinol should be discontinued at the first sign of rash or hypersensitivity and should not be restarted. Acute gout attacks may increase during the initial weeks of therapy; prophylactic colchicine or NSAIDs should be considered.

Drug Interactions

Allopurinol significantly increases the toxicity of azathioprine and mercaptopurine by inhibiting their metabolism; doses of these medications must be reduced by 50-75% when used concurrently. The combination with ampicillin or amoxicillin increases the risk of rash. Allopurinol may enhance the anticoagulant effect of warfarin. ACE inhibitors may increase the risk of hypersensitivity reactions. Thiazide diuretics may increase the risk of allopurinol hypersensitivity and may also contribute to hyperuricemia.

Special Populations

Allopurinol should be used during pregnancy only if clearly needed, as adequate human studies are lacking. The medication is excreted in breast milk; caution should be exercised when administering to nursing mothers. Safety and efficacy have not been established in pediatric patients except for those with hyperuricemia secondary to malignancy. Elderly patients may require lower doses due to age-related decreases in renal function. Dose reduction is required in patients with renal impairment; starting doses of 50-100 mg daily are recommended, with careful titration based on serum urate response and tolerability.

Frequently Asked Questions

Lowering serum urate quickly mobilizes urate from joint tissue deposits, which can trigger acute flares during the first 3 to 6 months of therapy. To prevent this, your doctor will typically prescribe low-dose colchicine or an NSAID for several months alongside allopurinol. Do not stop allopurinol if a flare occurs — continuing therapy is essential for long-term control.

The American College of Rheumatology recommends HLA-B*5801 screening for patients of Han Chinese, Korean, Thai, or African descent because this allele substantially increases the risk of severe cutaneous adverse reactions such as Stevens-Johnson syndrome and toxic epidermal necrolysis. Patients who test positive should avoid allopurinol and use an alternative urate-lowering therapy.

For most patients with gout, allopurinol is a lifelong medication. Stopping treatment generally allows serum urate to rise again and gout flares to recur within months. The goal is sustained serum urate below 6 mg/dL — or below 5 mg/dL for patients with tophi — which over time dissolves existing urate deposits.

Stop allopurinol and seek urgent medical care if you develop a rash accompanied by fever, blistering, mouth sores, eye redness, peeling skin, or facial swelling. These can be early signs of severe cutaneous reactions (SJS/TEN/DRESS) that carry significant mortality. A simple, mild rash without these features should still be reported to your doctor promptly.

Allopurinol itself does not directly interact with alcohol, but beer and spirits raise serum urate and trigger gout flares. Beer is particularly problematic because it contains purines in addition to alcohol. Limiting or avoiding alcohol — especially beer — substantially improves long-term gout control regardless of allopurinol therapy.

Reducing intake of high-purine foods (organ meats, anchovies, sardines, shellfish, and red meat) and sugar-sweetened beverages lowers urate production. Increasing low-fat dairy, cherries, coffee, and water intake has been shown to lower urate or reduce flare risk. Diet alone is rarely sufficient, but it complements allopurinol effectively.

Questions to Ask Your Doctor

Consider discussing these topics at your next appointment:

✓What serum urate target should we aim for, and how often will you check my level?
✓Should I take a flare-prevention medication when starting allopurinol, and for how long?
✓Are any of my current medications likely to interact with allopurinol?
✓Do I need HLA-B*5801 testing based on my ancestry?
✓What rash or systemic symptoms should make me stop allopurinol and call you?

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Medical Disclaimer: This information is for educational purposes only and should not be considered medical advice. Always consult with your healthcare provider before starting, stopping, or changing any medication. Your doctor can provide personalized recommendations based on your specific health condition and medical history.