Allopurinol
1,5-dihydro-4H-pyrazolo[3,4-d]pyrimidin-4-one
Also known as: Zyloprim · Aloprim · Zyloric · Zylapour · allopurinolum · BW-56-158 · HPP · NSC-1390 · oxypurinol (active metabolite) · alloxanthine
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Dogs
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Urate uroliths (dissolution) | 15 mg/kg PO q12h; only in conjunction with low purine foods. | PO | q12h | — | 🌎 NA |
| Urate uroliths (prevention) | 10-20 mg/kg/day | PO | Daily | — | 🌎 NA |
| Urate uroliths (dissolution and prevention) | 7-10 mg/kg PO three times daily | PO | TID | — | 🌎 NA |
| Leishmaniasis (First line treatment) | 10 mg/kg PO twice daily | PO | BID | 6-12 months | 🌎 NA |
| Leishmaniasis (Second line treatment) | 10 mg/kg PO twice daily | PO | BID | 6-12 months | 🌎 NA |
| Leishmaniasis | 20 mg/kg PO q12h | PO | q12h | 9 months | 🌎 NA |
| Leishmaniasis (Alternate protocol alone) | 10 mg/kg PO q8h or 10-20 mg/kg PO q12h | PO | q8h or q12h | 1-4 months | 🌎 NA |
| Leishmaniasis | 10 mg/kg PO twice daily | PO | BID | — | 🌎 NA |
| Leishmaniasis (with renal insufficiency) | 5 mg/kg PO twice daily | PO | BID | — | 🌎 NA |
| Leishmaniasis | 15 mg/kg PO twice daily | PO | BID | months | 🌎 NA |
| Uric acid urolithiasis (Dissolution) | 10 mg/kg p.o. q8h or 15 mg/kg p.o. q12h | PO | q8h or q12h | up to 4 weeks | 🌍 EU |
| Uric acid urolithiasis (Prevention) | 10-15 mg/kg | PO | q24h | — | 🌍 EU |
| Leishmaniosis | 10 mg/kg | PO | q12h | at least 6-12 months | 🌍 EU |
- Urate uroliths (prevention): Preferable to minimize recurrence with dietary therapy to avoid xanthine uroliths.
- Urate uroliths (dissolution and prevention): Goal is to reduce urine urate:creatinine ratio by 50%.
- Leishmaniasis (First line treatment): Used plus Meglumine antimoniate 75-100 mg/kg once daily for 4-8 weeks.
- Leishmaniasis (Second line treatment): Used plus Miltofosine 2 mg/kg PO once daily for 4 weeks OR allopurinol alone.
- Leishmaniasis: With Meglumine antimoniate (100 mg/kg/day SQ) until resolution.
- Leishmaniasis: If possible use with meglumine antimoniate, if not, use alone.
- Leishmaniosis: Use with meglumine antimonate for 1-2 months, or miltefosine for 1 month (NB: this does not result in complete parasitological cure).
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Leishmaniosis | 10-20 mg/kg | PO | q12-24h | — | 🌍 EU |
- Leishmaniosis: Has been shown to be effective (although experience is limited in this species).
Birds
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Gout (budgies and cockatiels) | Crush one 100 mg tablet into 10 mL of water. Add 20 drops of this solution to one ounce of drinking water. | PO | Continuous in water | — | 🌎 NA |
| Gout (parakeets) | Crush one 100 mg tablet into 10 mL of water. Add 20 drops of this solution to one ounce of drinking water or give 1 drop 4 times daily. | PO | Continuous or QID | — | 🌎 NA |
Reptiles
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Elevated uric acid levels in renal disease (lizards) | 20 mg/kg PO once daily | PO | SID | — | 🌎 NA |
| Gout | 20 mg/kg PO once daily | PO | SID | — | 🌎 NA |
| Hyperuricemia (green iguanas) | 25 mg/kg PO daily | PO | SID | — | 🌎 NA |
- Gout: Suggested dosage based upon human data as dose is not established for reptiles.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Allopurinol is a potent xanthine oxidase inhibitor widely used in veterinary medicine.
Key clinical applications include:
- Urate Urolithiasis: Used primarily in dogs (especially predisposed breeds like Dalmatians) to dissolve and prevent uric acid stones. It must be combined with a low-purine diet to prevent the formation of xanthine stones.
- Canine Leishmaniasis: Acts as a leishmaniostatic agent. It is often used in combination with meglumine antimoniate or miltefosine for long-term management.
- Avian and Reptilian Medicine: Used to manage hyperuricemia and treat gout in birds and reptiles.
Clinical Pearl: Because allopurinol increases the urinary excretion of xanthine, prolonged high doses without dietary purine restriction can paradoxically lead to the formation of xanthine uroliths.
Mechanism of action
Allopurinol and its primary active metabolite, oxypurinol (alloxanthine), competitively inhibit the enzyme xanthine oxidase.
- Normal Pathway: Hypoxanthine → xanthine oxidase → Xanthine → xanthine oxidase → Uric Acid
- Inhibition: By blocking this enzyme, allopurinol prevents the conversion of oxypurines to uric acid. This decreases serum and urine concentrations of uric acid, while increasing the levels of the more water-soluble precursors (hypoxanthine and xanthine).
In Leishmaniasis: Allopurinol acts as a purine analogue. Leishmania organisms mistakenly incorporate allopurinol metabolites into their RNA, which disrupts protein synthesis and inhibits parasite multiplication.
Safety & warnings
Contraindications
- Known hypersensitivity to allopurinol
- Red-tailed hawks (anecdotal reports of severe toxicity)
Adverse effects
- Xanthine urolithiasis (especially with high doses or non-restricted diets)
- Hypersensitivity reactions (rash, erythema)
- Lethargy
- Gastrointestinal upset
- Hepatotoxicity (rare)
- Bone marrow suppression (rare)
Precautions
Prolonged high doses of allopurinol may result in xanthine uroliths; it is highly recommended to use concurrently with a low-purine diet for urolith management. Use with caution and consider dose reduction (e.g., 5 mg/kg PO twice daily) in patients with renal insufficiency.
Drug interactions
May increase risks for hypoglycemia and hepato-renal reactions
Increased bone marrow depression may occur
Allopurinol may increase cyclosporine levels
Can increase uric acid levels, antagonizing allopurinol's effect
Allopurinol may reduce the metabolism of warfarin thereby increasing its effect
Thrombocytopenia has occurred in a few human patients when used concurrently
May increase the renal excretion of oxypurinol and thereby reduce xanthine oxidase inhibition; additive effects on blood uric acid may be beneficial
May reduce the solubility of uric acid in the urine and induce urolithiasis
Allopurinol inhibits the metabolism of azathioprine, leading to severe, potentially fatal myelosuppression.
Allopurinol inhibits the metabolism of mercaptopurine, significantly increasing toxicity risk.
Concurrent use increases the risk of cutaneous rash.
May decrease theophylline clearance, increasing the risk of toxicity.
May increase the risk of allopurinol hypersensitivity and toxicity.
Increased risk of skin rashes when used concurrently.
Monitoring
- Urine uric acid (for urolithiasis)
- Adverse effects (rash, lethargy)
- Periodic CBC, liver and renal function tests (e.g., BUN, Creatinine, liver enzymes); especially early in therapy
Pharmacokinetics
Absorption
Rapidly absorbed following oral administration.
Distribution
Widely distributed in tissue fluid; not highly bound to plasma proteins.
Metabolism
Rapidly oxidized to its primary active metabolite, oxypurinol (alloxanthine), which is also a xanthine oxidase inhibitor.
Elimination
Excreted primarily in the urine as oxypurinol and unchanged allopurinol.
Overdose
Information on acute overdosage in veterinary species is limited. Massive overdoses may cause gastrointestinal distress (vomiting, diarrhea). Treatment should consist of supportive care and monitoring of renal function. Adequate hydration is important to promote diuresis and excretion.
Available products
Formulations
- Oral tablets
- Powder for injection (lyophilized)
- Compounded oral suspension
Human-labeled
- Allopurinol Oral Tablets: 100 mg & 300 mg; Zyloprim® (Faro); generic
- Allopurinol Powder for Injection, lyophilized: 500 mg preservative-free in 30 mL vials; Aloprim® (Nabi); generic (Bedford Labs)
Regulatory status
Approved in several EU member states specifically for canine leishmaniosis.
Used under the cascade where specific veterinary licensed products are unavailable.
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Tablets should be stored at room temperature in well-closed containers; stable in light and air. Powder for injection should be stored at 25°C (excursions permitted to 15-30°C). Once diluted to ≤6 mg/mL, store at room temperature and use within 10 hours; do not refrigerate. Compounded oral suspensions (20 mg/mL) in Ora-Plus/Ora-Sweet retain >95% potency for 60 days at 5°C and 25°C and should be protected from light.
Client information
Important: Unless otherwise directed by your veterinarian, administer this medication after meals (usually about 1 hour after) to minimize stomach upset.
- Dietary Management: If your pet is being treated for urate stones, strictly follow the low-purine diet prescribed by your vet. Giving this medication without the proper diet can lead to the formation of a different type of stone (xanthine).
- When to Call the Vet: Notify your veterinarian immediately if your animal develops a skin rash, becomes unusually lethargic, or appears ill.
- Ensure your pet always has access to plenty of fresh drinking water to help flush the urinary system.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.