Allopurinol (Zyloprim®)

Overview

Allopurinol belongs to a class of drugs known as xanthine oxidase inhibitors. 

The enzyme xanthine oxidase is responsible for converting certain body chemicals to uric acid. 

The uric acid produced is normally changed to the chemical allantoin, which is then eliminated in the urine.

Some animals have inherited defects and cannot convert uric acid to allantoin. This is similar to the problem of gout in people. This problem is especially common in Dalmatian dogs. If uric acids (urates) accumulate in the body, urate crystals form in the urine. These crystals may congregate to produce kidney or bladder stones. 

Dalmatians, and pets with blood flow abnormalities to the liver (known as portosystemic shunts), are also at a higher risk for developing urate stones.

Allopurinol is a prescription drug and can only be obtained from a veterinarian or by prescription from a veterinarian.

This drug is not approved for use in animals by the Food and Drug Administration but it is prescribed legally by veterinarians as an extra-label drug.

Brand Names and Other Names

This drug is registered for use in humans only.

Human formulations: Zyloprim® (Glaxo) and generics

Veterinary formulations: None

Uses of Allopurinol

Allopurinol is most commonly used to prevent recurrent urate bladder stones.

Allopurinol has been used, in combination with other drugs, to treat a serious infection in animals caused by leishmania (canine leishmaniasis). Canine leishmaniasis has recently been shown to be a problem in the eastern United States. 

Precautions and Side Effects

While generally safe and effective when prescribed by a veterinarian, allopurinol can cause side effects in some animals.

Allopurinol should not be used in animals with known hypersensitivity or allergy to the drug.

It should be used with great caution in animals with impaired kidney and liver function.

Allopurinol may interact with other medications. Consult with your veterinarian to determine if other drugs your pet is receiving could interact with allopurinol. Such drugs include azathioprine, methionine, ammonium chloride and furosemide. 

There are few reported adverse effects of allopurinol in animals. In people, there have been reports of gastrointestinal upset, skin rashes and liver problems. 

If administered for a long period, there is a risk of developing unusual bladder stones called xanthine stones. 

How Allopurinol Is Supplied

Allopurinol is available in 100 mg and 300 mg tablets.

Dosing Information

Medication should never be administered without first consulting your veterinarian. 

In dogs, the usual dose is 3 to 5 mg per pound (10 mg/kg) every 8 hours or 7 mg per pound (15 mg/kg) every 12 hours. 

In cats, the usual dose is 4 mg per pound (8.8 mg/kg) per day.

For the treatment of canine leishmaniasis, allopurinol is dosed at 5 mg per pound (10 mg/kg) every 12 hours for prolonged treatment (for example 4 months).

The duration of administration depends on the condition being treated, response to the medication and the development of any adverse effects. Be certain to complete the prescription unless specifically directed by your veterinarian. Even if your pet feels better, the entire treatment plan should be completed to prevent relapse or prevent the development of resistance.