Allopurinol

What is allopurinol?

Allopurinol (Lopurin®, Zyloprim®) is a drug used to prevent the recurrence of uric acid and calcium oxalate uroliths (stones) in dogs. This medication works by decreasing the production of uric acid in the body.

It is used off label (extra label) to treat leishmaniasis in dogs and cats, and to treat gout in certain birds and reptiles. Many drugs are commonly prescribed for off-label use in veterinary medicine.

“Off label” means the drug is being used in a way that has not been reviewed or approved by the U.S. Food and Drug Administration (FDA) or Health Canada. In these instances, follow your veterinarian’s directions and cautions carefully, as they may be significantly different from those on the label. 

How do I give allopurinol to my pet?

• Allopurinol comes in tablet form and is given orally (by mouth). For birds, it may be added to the drinking water.
• Allopurinol may be given with or without food.
• If your pet vomits or seems unwell after receiving allopurinol on an empty stomach, give the next dose with a small amount of food. If vomiting continues, contact your veterinarian.
• Compounded liquid forms may be prepared by your veterinarian. The liquid form must be measured carefully. Your veterinarian can provide you with a syringe to measure the correct amount.
• Ensure your pet has free access to fresh water while taking this medication unless otherwise advised by your veterinarian.
• When used for the long-term prevention of stones in dogs, a low-purine diet should be fed.

This medication can take up to a few weeks before effects are noted and improvement may not always be visible.

What if I miss giving a dose to my pet?

If you miss a dose, give it as soon as you remember, and then continue with the regular dosing schedule. However, if it is almost time for the next dose, skip the missed dose and resume the regular dosing schedule. Do not give the pet two doses at once.

Are there any potential side effects?

Allopurinol may cause the following side effects:

• Vomiting, lack of appetite
• Diarrhea

In dogs receiving higher doses of allopurinol, certain types of bladder stones (xanthine stones) may develop, causing signs of urinary inflammation or obstruction, including difficulty urinating, bloody urine and urinary accidents.

Other side effects may occur. If you notice anything unusual, contact your veterinarian immediately.  This short-acting medication should stop working within 24 hours, although effects can be longer in pets with liver or kidney disease.

Are there any risk factors for this medication?

DO NOT USE allopurinol in:

• Red-tailed hawks

USE WITH CAUTION in:

• Animals with liver or kidney problems
• Animals that are breeding or pregnant (safety has not been determined)
• Dogs that are nursing
• Raptors other than red-tailed hawks

Are there any drug interactions I should be aware of?

Several medications should be used with caution when given with allopurinol, including:

• Certain antibiotics (e.g., amoxicillin, ampicillin)
• Immunosuppressive drugs
• Diuretics

Certain other medications may interact with allopurinol, so be sure to tell your veterinarian about any medications (including vitamins, supplements, or herbal therapies) that your pet is taking before starting any new treatment.

Is any monitoring needed with this medication?

• Your veterinarian may recommend monitoring of uric acid levels in the blood and urine.
• Your veterinarian may recommend liver and kidney function tests.
• If allopurinol is being used to treat leishmaniasis, your veterinarian will monitor the clinical signs of the disease.
• At home, monitor for signs of liver dysfunction such as yellowing of the skin, severe tiredness, or skin rash.
• Your dog should also be monitored for urinary problems (e.g., straining to urinate, blood in the urine, pain while urinating) while taking allopurinol, because bladder stones can develop with long-term use. Consult your veterinarian if any of these problems occur.

How do I store allopurinol?

• Store allopurinol in a tightly sealed container at room temperature, between 68°F and 77°F (20°C and 25°C). Protect it from light.
• If your veterinarian has compounded a liquid form, follow the storage recommendations and expiration date for the medication printed on the label.

What should I do in case of an emergency?

If you suspect an overdose or an adverse reaction to the medication, call your veterinary office immediately. If they are not available, follow their directions in contacting an emergency facility. Be prepared to provide information about the product and your pet's weight and symptoms.