Allopurinol

Allopurinol is a purine analogue used for two different veterinary purposes: reducing uric acid formation and suppressing clinical leishmaniasis. In dogs predisposed to urate uroliths, it changes the products of purine metabolism and is used alongside dietary management. In leishmaniasis, it acts on parasite metabolism and generally forms part of a longer-term combination strategy.

Clinical improvement in leishmaniasis is not equivalent to elimination of infection. Similarly, lowering uric acid does not remove all stone risk, because the less metabolized purines that accumulate during treatment can form xanthine uroliths.

Group

Xanthine oxidase inhibitor; antileishmanial

Mechanism of Action

Allopurinol and its active metabolite oxypurinol inhibit xanthine oxidase, the enzyme that converts hypoxanthine to xanthine and xanthine to uric acid. Less uric acid is produced and delivered to urine, reducing the substrate available for urate crystal formation. It acts by decreasing production, not by directly increasing renal uric acid excretion or physically dissolving an established obstruction.

In Leishmania, purine-salvage pathways convert allopurinol to abnormal nucleotides that interfere with RNA and protein synthesis. This suppresses multiplication and can improve disease manifestations, but usual treatment does not reliably eradicate tissue infection.

Xanthine oxidase inhibition also increases upstream purines, particularly xanthine. Because xanthine has limited urinary solubility, its accumulation explains treatment-associated xanthine crystalluria or calculi and the importance of the purine load in the overall treatment plan.

Dose Rates

Dogs

Use / stage	Dose	Route	Schedule	Instructions
Urate urolith prevention — Initial	10 mg/kg	PO	Every 8 h	Do not use in: Allopurinol hypersensitivity; Reduce to the maintenance schedule after the initial phase; transition timing is not specified.
Urate urolith prevention — Maintenance	10 mg/kg	PO	Every 24 h	Do not use in: Allopurinol hypersensitivity
Leishmaniasis	10 mg/kg	PO	Every 12 h	Do not use in: Allopurinol hypersensitivity; Treat for at least 4 months, commonly 4–6 months; some combination courses continue 6 months or longer.
Leishmaniasis - alternative regimen — Initial	15 mg/kg	PO	Every 12 h	Do not use in: Allopurinol hypersensitivity; If a response develops, change to the alternative maintenance schedule.
Leishmaniasis - alternative regimen — Maintenance	7–10 mg/kg	PO	Every 12–24 h	Do not use in: Allopurinol hypersensitivity
Urate urolith dissolution	15 mg/kg	PO	Every 12 h	Do not use in: Allopurinol hypersensitivity; Use with a low-purine diet.
Urate urolith dissolution - alternative	7–10 mg/kg	PO	Every 8 h	Do not use in: Allopurinol hypersensitivity; Low-purine diet and urine urate:creatinine monitoring.
Urate urolith prevention - alternative	7–10 mg/kg	PO	Every 8 h	Do not use in: Allopurinol hypersensitivity; Low-purine diet and urine urate:creatinine monitoring.
Urate urolith prevention - daily amount	10–20 mg/kg	PO		Do not use in: Allopurinol hypersensitivity; Total daily amount, not a specified single dose; division across the day is not established. Minimize chronic exposure because of xanthine-stone risk.
Leishmaniasis - prolonged course	10 mg/kg	PO	Every 12 h	Do not use in: Allopurinol hypersensitivity; 6–12 months. Options include meglumine antimoniate 75–100 mg/kg daily for 4–8 weeks, or miltefosine 2 mg/kg PO daily for 4 weeks; the antimonial route is not specified in this alternative course.
Leishmaniasis - antimonial combination	20 mg/kg	PO	Every 12 h	Do not use in: Allopurinol hypersensitivity; 9 months with meglumine antimoniate 100 mg/kg/day SC until resolution.
Leishmaniasis - short alternative	10 mg/kg	PO	Every 8 h	Do not use in: Allopurinol hypersensitivity; 1–4 months.
Leishmaniasis - short twice-daily alternative	10–20 mg/kg	PO	Every 12 h	Do not use in: Allopurinol hypersensitivity; 1–4 months.
Leishmaniasis with renal insufficiency	5 mg/kg	PO	Every 12 h	Do not use in: Allopurinol hypersensitivity; Reduced-dose alternative with intensified renal monitoring.

Horses

Use / stage	Dose	Route	Schedule	Instructions
Use not specified	5 mg/kg	PO		Do not use in: Allopurinol hypersensitivity; The equine dose is listed without an indication or repeat interval.

Lizard

Use / stage	Dose	Route	Schedule	Instructions
Hyperuricemia with renal disease	20 mg/kg	PO	Every 24 h	Do not use in: Allopurinol hypersensitivity

Reptile

Use / stage	Dose	Route	Schedule	Instructions
Gout	20 mg/kg	PO	Every 24 h	Do not use in: Allopurinol hypersensitivity; Suggested extrapolated regimen; species-specific pharmacokinetic validation is limited.

Green iguana

Use / stage	Dose	Route	Schedule	Instructions
Hyperuricemia	25 mg/kg	PO	Every 24 h	Do not use in: Allopurinol hypersensitivity

Budgerigar

Use / stage	Dose	Route	Schedule	Instructions
Gout	Reference only	PO		Do not use in: Red-tailed hawks, Allopurinol hypersensitivity; Disperse 100 mg in 10 mL water; add 20 drops to one ounce of drinking water. Parakeet alternative: one drop orally four times daily. Drop volume and actual water intake are not standardized.

Cockatiel

Use / stage	Dose	Route	Schedule	Instructions
Gout	Reference only	PO		Do not use in: Red-tailed hawks, Allopurinol hypersensitivity; Disperse 100 mg in 10 mL water; add 20 drops to one ounce of drinking water. Parakeet alternative: one drop orally four times daily. Drop volume and actual water intake are not standardized.

Parakeet

Use / stage	Dose	Route	Schedule	Instructions
Gout	Reference only	PO		Do not use in: Red-tailed hawks, Allopurinol hypersensitivity; Disperse 100 mg in 10 mL water; add 20 drops to one ounce of drinking water. Parakeet alternative: one drop orally four times daily. Drop volume and actual water intake are not standardized.

Indications

Prevention of recurrent urate urolith formation in predisposed dogs, including hyperuricosuric patients, alongside dietary and urinary management.
Suppression of clinical canine leishmaniasis, generally in combination with another antileishmanial agent, to improve cutaneous or systemic signs and support remission.
Long-term suppression of canine leishmaniasis after initial treatment; persistent infection and relapse remain possible.

Contraindications

Known hypersensitivity or a previous severe reaction.
Reduce dose and intensify monitoring in renal or hepatic dysfunction.
Contraindicated in red-tailed hawks; avoid or use extreme caution in other raptors.
Pregnancy safety is not established; use cautiously during lactation.

Interactions

Avoid azathioprine: impaired metabolism increases its toxicity.
Theophylline or aminophylline exposure may rise with high allopurinol doses.
Potential increased exposure or toxicity with cyclosporine, warfarin, chlorpropamide or cyclophosphamide.
Urinary acidifiers reduce uric-acid solubility and can promote calculi.
Amoxicillin or ampicillin has increased rash risk in people; veterinary significance is uncertain.
Uricosuric drugs increase oxypurinol elimination; trimethoprim/sulfamethoxazole has been associated with thrombocytopenia in people.

Adverse effects

Hypersensitivity skin reactions.
Gastrointestinal upset; xanthine uroliths can develop with prolonged treatment and a purine-rich diet.
Rare canine reports include hemolytic anemia and trigeminal neuropathy.

Toxicity

Overdose may cause vomiting and trembling. Renal dysfunction can increase exposure; assess renal function, hydration and clinical signs.

Preparations

Preparation	Form	Strength	Route
Allopurinol 100 mg tablet	Tablet	100 mg / tablet	PO
Allopurinol 300 mg tablet	Tablet	300 mg / tablet	PO

Monitoring and Administration

Assess clinical response during leishmaniasis treatment, including renal involvement and proteinuria. Combination treatment may include meglumine antimoniate 100 mg/kg SC daily for 4 weeks, with allopurinol 10 mg/kg PO every 12 hours for 6 months or longer.

Keep well closed at room temperature. Some compounded preparations remain stable for at least 60 days; solution stability is greatest at pH 3–3.4.

Use a low-purine diet during urate dissolution and consider dietary control to limit long-term xanthine-stone risk. Monitor urine urate, CBC, liver and renal indices; one treatment target is a 50% reduction in urine urate:creatinine ratio. Urate dissolution is unlikely to work with portovascular anomalies.

A 20 mg/mL suspension prepared with equal volumes Ora-Plus and Ora-Sweet or Ora-Sweet SF retains potency for 60 days at 5°C or 25°C. A Cologel/syrup-based 20 mg/mL preparation has a separate 14-day stability period. Protect compounded liquids from light.