Allopurinol

Xanthine oxidase inhibitor Uricosuric agent (antihyperuricemic)

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog PO 10-20 mg/kg q12h Duration: Long-term (months to years) for urolithiasis; for leishmaniasis: 10 mg/kg q12h for 6-12 months
Notes: For urate urolithiasis, combine with low-purine diet and urinary alkalinization. For leishmaniasis, use in combination with antimonials.
Cat PO 9 mg/kg q24h Duration: Long-term
Notes: Use cautiously in cats with renal disease; adjust dose as needed.
Horse PO 5-10 mg/kg q12h Duration: Variable
Notes: Limited evidence; use only under veterinary supervision.
Rabbit PO 10-20 mg/kg q12h Duration: Long-term
Notes: Use with caution; monitor renal function.
Reptiles PO 10-20 mg/kg q24h Duration: Variable
Notes: For visceral gout; ensure adequate hydration.

Clinical Indications & Species Uses

General Indications
  • Prevention of urate urolithiasis
  • Management of hyperuricemia
Dog (Canine)
  • Urate urolithiasis (prevention and treatment)
  • Hyperuricemia (e.g., due to portosystemic shunt, Dalmatian breed)
  • Leishmaniasis (as adjunct therapy)
  • Gout (rarely in dogs)
Cat (Feline)
  • Urate urolithiasis (prevention and treatment)
  • Hyperuricemia (rarely used)
Horse (Equine)
  • Hyperuricemia (rarely used)
  • Equine polysaccharide storage myopathy (experimental)
Rabbit & Small Mammals
  • Urate urolithiasis (occasionally used)
Exotic & Other Species
  • Reptiles: visceral gout (occasionally used)

Pharmacology & Mechanism of Action

Drug Class: Xanthine oxidase inhibitor | Pharmacological Group: Uricosuric agent (antihyperuricemic)

Mechanism of Action: Allopurinol and its active metabolite oxypurinol inhibit xanthine oxidase, the enzyme responsible for converting hypoxanthine to xanthine and xanthine to uric acid. By reducing uric acid production, allopurinol decreases serum and urinary uric acid concentrations, preventing urate crystal deposition and urolith formation.

Pharmacodynamics: Allopurinol reduces serum uric acid levels within 24-48 hours, with maximal effect in 1-2 weeks. It also decreases urinary uric acid excretion, which is beneficial in preventing urate uroliths. In dogs with leishmaniasis, allopurinol has immunomodulatory effects and inhibits purine metabolism of the Leishmania parasite.

⚡ Pharmacokinetics Summary

Absorption: Rapidly and well absorbed after oral administration in most species. Bioavailability is approximately 80-90% in dogs and humans.
Distribution: Widely distributed throughout body tissues and fluids. Crosses the placenta and is excreted in milk. Volume of distribution is approximately 1.6 L/kg in dogs.
Metabolism: Metabolized primarily in the liver to oxypurinol (alloxanthine), which is also pharmacologically active. Oxypurinol has a longer half-life than the parent compound.
Excretion: Excreted primarily via the kidneys, both as unchanged allopurinol and as oxypurinol. Renal clearance is significant; dose adjustment is needed in renal impairment.
Half-Life: Dog: allopurinol ~1-2 hours, oxypurinol ~15 hours; Cat: allopurinol ~1-3 hours, oxypurinol ~20 hours; Horse: allopurinol ~1-2 hours; Cattle: ~2-4 hours.
Bioavailability: Oral bioavailability is approximately 80-90% in dogs and cats.
Protein Binding: Low, approximately <5% for allopurinol and oxypurinol.

Available Formulations & Strengths

Oral Tablet 100 mg, 300 mg (PO)
Oral Suspension 20 mg/mL (compounded) (PO)
Injectable Solution (IV) 500 mg/30 mL (human formulation, not commonly used in vet med) (IV)

Contraindications & Clinical Warnings

Contraindications:
  • Hypersensitivity to allopurinol or any component
  • Severe renal impairment (unless dose adjusted)
  • Concurrent use with azathioprine or mercaptopurine (unless dose reduction of these drugs)
  • Pregnancy (unless benefits outweigh risks)
Warnings & Clinical Precautions:
  • Use with caution in patients with renal or hepatic impairment
  • Monitor serum uric acid levels periodically
  • Ensure adequate water intake to prevent xanthine urolithiasis
  • In dogs with leishmaniasis, allopurinol is not curative alone; combine with antileishmanial drugs
  • Discontinue if skin rash or hypersensitivity occurs
  • May cause drowsiness; caution when driving or operating machinery

Adverse Effects & Reactions

Common:

  • Gastrointestinal upset (vomiting, diarrhea)
  • Drowsiness
  • Skin rash

Serious / Severe:

  • Hepatotoxicity
  • Bone marrow suppression (leukopenia, thrombocytopenia)
  • Acute renal failure
  • Severe hypersensitivity reactions (Stevens-Johnson syndrome)

Rare:

  • Peripheral neuropathy
  • Alopecia
  • Cataracts (in rats, not confirmed in domestic animals)

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
Azathioprine Allopurinol inhibits metabolism of azathioprine, increasing risk of bone marrow suppression. Reduce azathioprine dose to 25% of usual. High
Mercaptopurine Similar to azathioprine; increased toxicity. High
Amoxicillin Increased risk of skin rash. Moderate
ACE inhibitors (e.g., enalapril) Increased risk of hypersensitivity reactions. Moderate
Thiazide diuretics May increase risk of renal toxicity and hypersensitivity. Moderate
Warfarin Allopurinol may increase anticoagulant effect; monitor coagulation. Moderate
Cyclophosphamide Increased risk of bone marrow suppression. Moderate

Overdose & Toxicity Management

Signs of Toxicity:

  • Nausea
  • Vomiting
  • Diarrhea
  • Dizziness
  • Hepatotoxicity
  • Renal failure

Emergency Treatment Protocol: Induce emesis if recent ingestion. Administer activated charcoal. Provide supportive care: IV fluids, monitoring of liver and renal function. There is no specific antidote. Treat symptoms as needed.

Food Animal Withdrawal Times

Not approved for food animals in the US; extra-label use requires extended withdrawal times. Consult FARAD for specific recommendations.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature 20-25°C (68-77°F)

Handling & Special Conditions: Protect from moisture. Keep container tightly closed.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Not FDA-approved for veterinary use; approved for human use.

Extra-Label (Off-Label) Use: In the US, allopurinol is not FDA-approved for veterinary use; use is extra-label. Must follow AMDUCA regulations for food animals, including extended withdrawal times.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Allopurinol is primarily used in dogs for the prevention of urate uroliths, especially in breeds like Dalmatians and in animals with portosystemic shunts. It is also used as an adjunct in the treatment of leishmaniasis. In cats, it is less commonly used but may be considered for urate uroliths. For urolithiasis, dietary management (low purine, alkalinizing diet) and increased water intake are essential. Monitor serum uric acid and urine pH. In leishmaniasis, allopurinol is often combined with meglumine antimoniate or miltefosine. Adverse effects are generally mild but can be serious; discontinue if hypersensitivity occurs. Dose adjustment is necessary in renal impairment. Always use with caution in pregnant or lactating animals.

References & Literature

  • 📚 Plumb's Veterinary Drug Handbook
  • 📚 Papich Veterinary Pharmacology and Therapeutics
  • 📚 Compendium of Veterinary Products (CVP)