Introduction

Here, we’ve put together the comprehensive guide on the factors to consider when dosing allopurinol; a drug used for the following indications:

• Gout prophylaxis
• Prevention of uric acid / calcium oxalate renal stones
• Prevention of hyperuricemia / tumor lysis syndrome in chemotherapy

Allopurinol is available in both oral and IV forms.

Please note:

• For pediatric doses, consult relevant guides based on age / weight.
• Dose adjustments are required in patients with either kidney or liver damage.

As part of the NAPLEX exam, and every other form of clinical pharmacy test, you are expected to have the practical prescribing knowledge that every experienced healthcare professional is expected to know.

Here, we provide that core detail – this time, on allopurinol.

Let’s get started.

Adult dose for Gout

Commence therapy on a low dose – 100mg per day.

• Titrate upward depending on the patient’s serum uric acid concentration. A standard maintenance dose is typically 200-300mg per day.
• Gradual titration required to avoid precipitation of acute gout attacks.
• Where doses of 300mg per day are required, divided doses are used to reduce gastric irritation.
• In severe tophaceous gout, doses of up to 600mg per day are not uncommon.
• Maximum dose: 800mg per day.
• Within 3 weeks, normal serum uric acid levels should be achieved.

Doses should be taken after food to avoid the effects of gastric irritation.

Hyperuricemia in Chemotherapy

Allopurinol is not always used for hyperuricemia secondary to chemotherapy. Other treatments – such as urate oxidase therapy – is often preferred. Nonetheless, allopurinol remains a viable treatment option. Chemotherapy regimens can rapidly produce states of hyperuricemia.

Allopurinol is preferably commenced 1-2 days before chemotherapy treatment.

• IV treatment: 200-400mg/m2/day as either a single infusion or in divided doses.
• Oral treatment: 600-800mg orally in divided doses. Maintenance doses are determined based on uric acid levels. Maximum dose of 800mg per day.

As before, oral doses in excess of 300mg per day should be given as divided doses.

Uric acid / Calcium Oxalate Renal Stones

Standard dose: 200-300mg per day either in single or divided doses.

Higher doses should be taken as divided doses to reduce gastric irritation.

Prescribing Factors to Consider

When prescribing allopurinol, bear the following factors in mind:

• When allopurinol is prescribed for gout, it’s not uncommon for it to be co-prescribed with an NSAID and/or colchicine. Treatment should continue well beyond 1-month after serum uric acid levels have returned to normal. This is done to ensure that the patient does not experience a sudden acute attack.
• Adequate hydration is necessary. Fluid intake of 2-3 liters is recommended. Desired outcome: neutral or alkaline urine.
• Dose of mercaptopurine and azathioprine must be reduced when taking allopurinol. That’s because allopurinol works as a xanthine oxidase inhibitor; the same enzyme that metabolizes mercaptopurine and azathioprine. As a result, levels of mercaptopurine and azathioprine should be substantially reduced – often as much as one-quarter of the standard dose.
• If switching from a uricosuric drug – such as probenecid – the following approach should be considered: gradually reduce the dose of probenecid whilst gradually increasing the dose of allopurinol.
• As we have discussed, oral doses greater than 300mg daily should be administered in divided doses and/or with food, to reduce gastric irritation.
• Patients should be informed that they should discontinue the drug if they experience a skin rash or any other signs of an allergic reaction.
• Patients may experience drowsiness which may interfere in their use of machinery. Counsel as appropriate.

Allopurinol is an important medicine. It is used to prevent (not treat) acute attacks of gout. It is also used to prevent uric acid / calcium oxalate renal stones, and to prevent hyperuricemia and tumor lysis syndrome in patients undergoing chemotherapy. Here, we’ve put together some of the essential prescribing factors that you, as a NAPLEX exam taker, need to know.

NAPLEX Study Guide is the leading online resource that helps pharmacy graduates become licensed, professional US pharmacists. Check back to our NAPLEX blog soon for even more practical prescribing tools to aid you in your long-term understanding of drugs, medicines and healthcare.