By Kayla Hyland, DVM; Rania Gollakner, BS, DVM, MPH

What is azathioprine?

Azathioprine (brand names: Imuran®, Azasan®) is an immune suppressing drug used to treat various autoimmune diseases in dogs and occasionally ferrets. It is typically not recommended for cats as the adverse effects can be severe, but it may occasionally be used.

Its use in dogs, cats, or ferrets to treat autoimmune diseases is “off label” or “extra label”. Many drugs are commonly prescribed for off-label use in veterinary medicine. In these instances, follow your veterinarian’s directions and cautions very carefully as their directions may be significantly different from those on the label.

How is azathioprine given?

Azathioprine is given by mouth as a tablet or compounded liquid suspension, or into the vein as an injectable solution in the hospital. When given by mouth, it can be given with or without food. If vomiting occurs when dosed on an empty stomach, give future doses with food or a treat. Do not stop this medication suddenly; it should be slowly tapered to avoid negative effects.

When administering this medication, gloves should be worn. Thoroughly wash your hands after giving the medication. Pregnant women should not handle this medication.

Azathioprine should take effect after several doses, but its full effect may take several weeks. Improvement in clinical signs may not be noticeable and so laboratory tests may be required to evaluate this medication’s effectiveness.

What if I miss giving my pet the medication?

If you miss a dose, give it when you remember. However, if it is close to time for the next dose, skip the dose you missed, give it at the next scheduled time, and return to the regular dosing schedule. Never give your pet two doses at once or give extra doses

Are there any potential side effects?

The most common side effects in dogs include vomiting, diarrhea, lack of appetite, bone marrow suppression leading to decreased red and white blood cells, pancreatitis, and liver toxicity. The most common side effect in cats is bone marrow suppression.

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?

Pets that are allergic to azathioprine should not use this medication. It should be used cautiously in patients with current infections or with liver or kidney disease. Avoid in pets with a history of pancreatitis or breeds at high risk of developing pancreatitis (e.g., schnauzers). Azathioprine is not recommended for cats due to the severity of adverse effects. When using this medication long term, there may be an increased risk for infection or tumor growth.

Azathioprine should not be used in pregnant or lactating animals unless it is the last resort for treatment. If used in lactating animals, milk replacer should be used.

Are there any drug interactions I should be aware of?

The following medications should be used with caution when given with azathioprine: ACE inhibitors, allopurinol, aminosalicylates, non-depolarizing muscle relaxants, corticosteroids, drugs affecting myelopoiesis (e.g., trimethoprim/sulfa), and warfarin.

Be sure to tell your veterinarian about any medications (including vitamins, supplements, or herbal therapies) that your pet is taking.

Is there any monitoring that needs to be done with this medication?

Complete blood counts should be monitored throughout treatment, from every 1 to 2 weeks to every 1 to 2 months, depending on length of treatment. Liver enzymes should also be measured prior to starting treatment and throughout treatment. Your veterinarian may monitor your pet to be sure that the medication is working.

How do I store azathioprine?

Powder for injection and tablets should be stored at room temperature between 15°C and 25°C (59°F to 77°F) and protected from light and moisture. Once the powder has been reconstituted, the medication should be used within 24 hours.

What should I do in case of 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 for contacting an emergency facility.

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