Allopurinol is used to lower uric acid production in conditions where uric acid has already accumulated, such as gout, tophi in the skin, and kidney stones. It is also indicated for the management of 2,8-dihydroxyadenine (2,8-DHA) renal stones associated with deficiency of adenine phosphoribosyltransferase. Additionally, it is used in patients with recurrent calcium oxalate kidney stones accompanied by high uric acid levels, particularly when dietary and fluid management measures are insufficient.

Allopurinol is an oral xanthine oxidase inhibitor that reduces uric acid formation by interfering with purine metabolism without affecting purine synthesis. It works by blocking the conversion of hypoxanthine to xanthine and xanthine to uric acid. Structurally, it resembles hypoxanthine, a natural purine compound. About 90% of the drug is absorbed from the gastrointestinal tract. Peak plasma levels are typically reached within 1.5 to 4.5 hours, and its half-life ranges from 1 to 2 hours. Around 20% of the administered dose is excreted in the feces.

Adults: Allopurinol should be introduced at low dosage e.g. 100 mg/day to reduce the risk of adverse reactions and increased only if the serum urate response is unsatisfactory. Extra caution should be exercised if renal function is poor. The following dosage schedules are suggested:

• 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.

Children: Children under 15 years: 10 to 20 mg/kg body weight/day up to a maximum of 400 mg daily. Use in children is rarely indicated, except in malignant conditions (especially leukaemia) and certain enzyme disorders such as Lesch-Nyhan syndrome.

Elderly: In the absence of specific data, the lowest dosage which produces satisfactory urate reduction should be used.

Dosage in renal impairment: In severe renal insufficiency, it may be advisable to use less than 100 mg per day or to use single doses of 100mg at longer intervals than one day.

When Allopurinol is used together with 6-mercaptopurine or azathioprine, the dose of these drugs should be reduced to about one-quarter of the usual amount, as Allopurinol prolongs their activity by inhibiting xanthine oxidase. It may also increase the half-life of vidarabine, potentially enhancing its toxic effects. Monitoring of theophylline levels is recommended when starting or adjusting Allopurinol therapy. A higher incidence of skin rashes has been observed in patients taking ampicillin or amoxicillin along with Allopurinol. Additionally, concurrent use with ciclosporin may increase its plasma concentration.

Allopurinol should not be used in patients who have a known hypersensitivity to the drug.

Possible side effects include skin rashes, gastrointestinal disturbances, and occasionally malaise, headache, dizziness, drowsiness, visual or taste disturbances, high blood pressure, hair loss, liver toxicity, nerve-related problems, gynecomastia, and blood disorders.

There is insufficient data regarding the safety of Allopurinol during pregnancy. It should only be used if no safer alternatives are available and the condition poses a significant risk to the mother or fetus. There is no adequate information on its effects during breastfeeding.

Allopurinol should be discontinued immediately if skin rash or any sign of hypersensitivity appears. Dose adjustment is necessary in patients with liver or kidney impairment. Caution is advised in patients with hypertension or heart failure, as they may also have reduced kidney function.

High doses of allopurinol (up to 22.5 g) have been reported without severe toxicity, although symptoms such as nausea, vomiting, diarrhea, and dizziness may occur at lower overdose levels. Adequate fluid intake helps enhance elimination of the drug and its metabolites. In severe cases, haemodialysis may be required.

Medications used for the treatment of gout.

Store below 30°C in a dry place, protected from light and moisture. Keep out of reach of children.