Azathioprine for Dogs: Uses, Dosage, and Side Effects

PetMD’s medications content was written and reviewed by veterinary professionals to answer your most common questions about how medications function, their side effects, and what species they are prescribed for. This content shouldn’t take the place of advice by your vet. 

Overview

Azathioprine is an immunosuppressant used in dogs to treat conditions where the body’s defenses attack its own tissues—known as inflammatory or immune mediated diseases, like immune-mediated hemolytic anemia, inflammatory bowel disease, and perianal fistulas. 

Azathioprine is often prescribed as an “add on” medication, if necessary, alongside ​certain medications such as ​p​​rednisone or ​​c​​yclosporine to help control inflammation. 

Is Azathioprine FDA-Approved for Dogs? 

Azathioprine is FDA-approved for human use under the brand names Imuran and Azasan. 

Azathioprine is currently not FDA-approved for use in dogs. However, it is commonly used in veterinary medicine, and veterinarians can legally prescribe certain human drugs for animals in specific circumstances. This is known as extra-label or off-label use because this use is not described on the drug label. 

Speak with your veterinarian about whether azathioprine is right for your dog. 

Compounded Azathioprine 

In certain circumstances, your veterinarian may recommend a compounded formulation of azathioprine. 

Compounded medications may be prescribed if there’s a specific reason your pet’s health can’t be managed by an FDA-approved drug, such as if: 

• Your pet has trouble taking the commercially available form 

• The required dosage strength is not available 

• Your pet is allergic to an ingredient in the FDA-approved medication 

Compounded medications are not FDA-approved. They are prepared by a veterinarian or licensed pharmacist on an individual basis to meet a pet’s specific needs. 

Considerations

This medication should be used with caution in dogs with:  

• Liver disease 

• Bone marrow disorders 

• History of pancreatitis 

Dogs on azathioprine may be at increased risk for infections and should be monitored closely. 

Giving azathioprine with certain medications can result in health risks to your dog, so it’s important to discuss with your veterinarian your dog’s medical condition and medications, including vitamins and supplements. 

How It Works

Azathioprine works by suppressing the immune system, which reduces inflammation caused by immune-mediated diseases. 

Azathioprine does this by interfering with the genetic information (DNA) in white blood cells, resulting in a decreased immune response. 

Directions

Follow the directions on the drug label or as provided by your veterinarian. 

• Azathioprine is typically given by mouth once daily at the start of treatment. 

• Your veterinarian may gradually reduce the dose over time to the lowest effective amount. 

• This medication can be given with or without food. 

• Consider wearing gloves and wash your hands after handling the medication. 

What Happens If You Miss a Dose of Azathioprine? 

Speak with your veterinarian about what to do if a dose is missed. 

Generally, your vet may instruct you to give it when you remember, or if it is almost time for your dog’s next dose, to skip the missed dose and resume your normal dosing schedule. 

Do not give extra or double doses. 

Possible Side Effects

Azathioprine can cause side effects related to a weakened immune system and organ toxicity. 

Common side effects may include: 

• Vomiting 

• Diarrhea 

• Loss of appetite 

• Abdominal pain 

• Weakness 

Serious side effects may include: 

• Pale gums 

• Abnormal bleeding or bruising 

• Yellowing (jaundice) of the skin or eyes 

• Fever 

If you believe your dog may be experiencing side effects, contact your veterinarian. 

Human Side Effects 

Humans should never take medication prescribed for pets, and pets should not be given medication prescribed for humans. Dosages often differ significantly. 

Long-term use of this medication may increase the risk of certain types of cancer in people. Consider wearing gloves during administration and wash hands after handling. Avoid contact if pregnant or nursing. 

If you accidentally ingest a pet medication, seek medical attention or call Poison Control at 800-222-1222. 

Monitoring Your Pet 

Because this medication can cause low blood counts and elevated liver ​​enzyme​ ​levels, your veterinarian may recommend routine testing depending on your dog’s individual needs, other medications, and the condition being treated. 

When Should You Call Your Veterinarian? 

Call your veterinarian if you see any of the following, or if you have additional questions or concerns about azathioprine:    

• Severe side effects 

• Condition worsens or does not improve 

• Suspected overdose 

Overdose Info

Symptoms of overdose may include vomiting, loss of appetite, and abnormal lab work findings, including liver enzyme elevations, and evidence of bone marrow suppression (abnormally low blood cell counts). 

If you suspect an overdose, seek emergency veterinary care or contact an animal poison control center. Consultation fees may apply. 

• Pet Poison Helpline: 855-764-7661 

• ASPCA Animal Poison Control: 888-426-4435 

Storage

Keep the following storage tips in mind for this medication: 

• Store at 68–77 F (20–25 C). 

• Keep ​the ​container tightly closed. 

• Protect from moisture and light. 

• Store compounded medications according to pharmacy instructions. 

• Keep out of reach of children and pets.