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Allopurinol

Allopurinol: Generic Medication Overview

Allopurinol is a prescription medication classified as a xanthine-oxidase inhibitor. It is available in pill form, most commonly in a 300 mg strength. In Hong Kong, the Drug Office of the Department of Health regulates its approval and distribution. Allopurinol is used to lower uric acid levels in the body, which helps prevent gout attacks and complications of hyperuricemia. It is not an analgesic; it works by addressing the biochemical cause of gout-related pain rather than providing direct pain relief.

How Allopurinol Works in the Body

Allopurinol blocks the enzyme xanthine oxidase, which is responsible for converting purines (substances found in many foods) into uric acid. By inhibiting this enzyme, the drug reduces the production of uric acid, leading to lower serum urate concentrations. The reduction in uric acid helps prevent the formation of monosodium urate crystals that deposit in joints and tissues, the primary trigger of gout flares. Onset of uric acid lowering usually occurs within a few days, with maximal effect reached after several weeks of consistent therapy. Allopurinol is well absorbed from the gastrointestinal tract, and its metabolites are eliminated primarily by the kidneys.

Conditions Treated by Allopurinol

• Gout - chronic management to prevent recurrent attacks and tophi formation.
• Hyperuricemia associated with chemotherapy or tumour lysis syndrome, where rapid cell turnover can raise uric acid levels.
• Kidney stones composed of uric acid, when dietary or other measures are insufficient.

Allopurinol is not indicated for immediate pain relief; it works over time to reduce the frequency and severity of gout attacks.

Patient Suitability and Contraindications

Who Should Use Allopurinol?

• Adults with documented hyperuricemia or a history of gout flares.
• Patients undergoing chemotherapy who are at risk of tumour lysis syndrome.

Absolute Contraindications

• Known hypersensitivity to allopurinol or any component of the tablet.
• Severe renal impairment (e.g., dialysis-dependent) where dosing cannot be safely adjusted.

Relative Contraindications

• Mild to moderate renal dysfunction - dose reduction and close monitoring are required.
• Hepatic impairment - monitor liver function tests.
• Pregnancy or breastfeeding - use only if the potential benefit outweighs the risk; allopurinol is classified as Category B2 in Hong Kong.

Special Populations

• Elderly: Start with a lower dose and increase slowly to minimise the risk of rash.
• Renal impairment: Dose is often reduced to 100 mg or 150 mg daily, depending on creatinine clearance.
• Hepatic disease: No specific dosage adjustment, but liver function should be checked periodically.

Safety Profile: Side Effects and Interactions

Common Side Effects

• Mild rash or itching - reported in many patients.
• Gastrointestinal discomfort (nausea, stomach upset).
• Elevated liver enzymes - usually transient.

Serious Adverse Events

• Severe cutaneous adverse reactions (SCARs): Stevens-Johnson syndrome or toxic epidermal necrolysis, though rare, require immediate medical attention.
• Agranulocytosis: Very uncommon, characterised by low white-blood-cell counts.
• Hepatotoxicity: Persistent elevation of liver enzymes may indicate liver injury.

Drug Interactions

• Azathioprine/6-mercaptopurine: Allopurinol inhibits their metabolism, potentially leading to bone-marrow suppression. Dose of the immunosuppressants should be reduced under specialist guidance.
• Warfarin: May increase INR; clotting parameters must be monitored more closely.
• Theophylline: Allopurinol can raise plasma theophylline levels, increasing the risk of toxicity.
• Cyclophosphamide and other chemotherapeutic agents: Enhanced risk of myelosuppression; dosing adjustments are recommended.

Food and Lifestyle Interactions

• High-purine foods (red meat, seafood, organ meats) can counteract the uric-lowering effect; a diet low in purines supports therapy.
• Alcohol, especially beer, may increase uric acid production and should be limited.
• No specific restrictions on caffeine, but excessive intake can exacerbate gout risk.

How to Take Allopurinol

• Standard dosing: The tablet is available in a 300 mg strength. Initial doses typically start at 100 mg once daily, with gradual titration toward 300 mg as tolerated and as indicated by serum urate levels.
• Administration: Swallow the tablet whole with a glass of water. It may be taken with or without food, but taking it with meals can reduce stomach upset.
• Missed dose: Take the missed dose as soon as remembered unless it is near the time of the next scheduled dose; do not double-dose.
• Overdose: Symptoms may include nausea, vomiting, diarrhoea, and dizziness. Seek emergency medical care; supportive care and monitoring are the mainstays of treatment.
• Discontinuation: Do not stop abruptly without consulting a healthcare professional, as sudden cessation can precipitate a gout flare. If discontinuation is necessary, a gradual taper may be advised.

Monitoring and Follow-Up

• Serum urate: Check 2-4 weeks after initiating therapy, then at regular intervals (every 3-6 months) to ensure target levels (<6 mg/dL in most patients).
• Renal function: Baseline and periodic serum creatinine measurements are recommended, especially in patients with pre-existing kidney disease.
• Liver enzymes: Periodic assessment is prudent, particularly when high doses are used.
• Complete blood count: Consider monitoring if the patient is also receiving azathioprine, mercaptopurine, or chemotherapy.

Storage and Handling

• Store tablets at room temperature (15-30 °C), protected from moisture and direct sunlight.
• Keep the container tightly closed and out of reach of children.
• Do not use the medication after the expiry date printed on the package.
• If a tablet becomes discolored or broken, discard it and obtain a new supply.

Medication-Specific Glossary

Xanthine oxidase

An enzyme that catalyzes the final steps in purine metabolism, producing uric acid as a by-product.

Hyperuricemia

An elevated concentration of uric acid in the blood, which can lead to gout and kidney stones.

Tophus

A deposit of monosodium urate crystals in soft tissue, often occurring in chronic gout.

Medical Disclaimer

This article provides educational information about allopurinol and is not a substitute for professional medical advice. Treatment decisions, including use for unapproved indications, must be made under the guidance of a qualified healthcare provider. The content is intended for informational purposes and does not constitute medical recommendations. Always consult a physician before starting, stopping, or changing any medication regimen.