Azathioprine for Dogs

Labrador dog lying next to bottle of pills and medication, close-up
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Azathioprine is a prescription drug used to treat certain autoimmune disorders in dogs. A healthy immune system works to destroy perceived threats such as bacteria, viruses, and other harmful substances. Autoimmune (or immune-mediated) disorders cause the immune system to become overactive and begin to attack the body's cells and tissues inappropriately. Treatment for autoimmune disorders generally involves using medications like azathioprine to suppress the overactive immune system.

What Does Azathioprine Do?

Azathioprine is an immunosuppressant drug that is also known by the brand name Imuran. It works by suppressing cells that produce antibodies, minimizing the body's immune response. This makes azathioprine an effective treatment for autoimmune disorders.

Azathioprine hinders the formation of purines, chemical compounds necessary to create DNA and RNA within cells. DNA is an essential part of cell replication and division in the body. Without it, cells cannot reproduce. Azathioprine is especially good at interrupting rapid cell division, like that which occurs within the immune system. It basically inhibits the body's ability to rapidly produce the cells that fight any perceived threats to the body. This is why it works well to suppress an overactive immune system. However, it can also leave the immune system vulnerable to valid threats, like germs.

Your veterinarian may prescribe azathioprine along with corticosteroids like prednisolone (which also helps suppress the immune system). In many cases, the goal of adding azathioprine to the treatment plan is to reduce the steroid dosage as much as possible. This is because the potential complications from steroid use are often worse than those associated with azathioprine use.

Azathioprine therapy is typically begun as a once-daily dose and tapered down to every-other-day administration. It should not be stopped suddenly unless specifically recommended by your veterinarian. Because the drug can be absorbed through the skin, it's important to wear gloves when handling it. Pregnant women and immunocompromised people should not be exposed to the drug at all.

Disorders Azathioprine Can Treat

There are several diseases in which azathioprine is a potentially effective treatment.

  • Immune-mediated thrombocytopenia (ITP; autoimmune platelet destruction)
  • Immune-mediated hemolytic anemia (IMHA; autoimmune red blood cell destruction)
  • Immune-mediated polyarthritis (rheumatoid arthritis)
  • Chronic active hepatitis (a type of liver disease)
  • Inflammatory bowel disease (severe cases)
  • Myasthenia gravis (autoimmune destruction of nerve/muscle junctions)
  • Systemic lupus erythematosus
  • Pemphigus foliaceus and other autoimmune skin diseases
  • Certain types of cancer

Side Effects of Azathioprine Use

Potential side effects of azathioprine treatment in dogs include the following:

  • Vomiting
  • Diarrhea
  • Loss of appetite
  • Lethargy
  • Fever
  • Pale appearance to gums and other mucous membranes
  • Yellowing of gums and other mucous membranes (jaundice)
  • Bruising and/or bleeding (blood in urine, nosebleeds, blood in stool)
  • Pancreatitis
  • Liver toxicity
  • Infections (due to suppression of the immune system)
  • Bone marrow suppression

Bone Marrow Suppression

In a healthy dog, bone marrow generates new blood cells. When there is bone marrow suppression, the body is unable to produce adequate numbers of new blood cells. This can lead to anemia (low red blood cells), leukopenia (low white blood cells), thrombocytopenia (low platelets). These blood cells have many important functions in the body. Inadequate numbers can lead to problems with organ function, blood clotting issues, and a weakened immune system (leaving them susceptible to infections).

Dogs on azathioprine will need to be monitored closely, especially in the early stages of use. Your vet will frequently check blood work to look for signs of bone marrow suppression and other complications.

Considerations Before Using Azathioprine for Dogs

Any medication can have side effects and other complications. A drug like azathioprine, that suppresses immune function, can come with major risks. However, if your veterinarian has prescribed azathioprine for your dog, then she feels the benefits outweigh the risks. The disease being treated may be more dangerous than the potential side effects.

Azathioprine should be used with caution (or not at all) in dogs with one or more of the following conditions:

  • Anemia (low red blood cell count)
  • Leukopenia (low white blood cell count)
  • Thrombocytopenia (low blood platelet count)
  • Infections
  • Blood clots
  • Pancreatitis (chronic or acute)
  • Pregnancy
  • Malignant lymphoma (cancer of the lymphatic system)
  • Kidney disease

Make sure your vet knows what other medications you are giving your pet, as some may interact with azathioprine or increase the risks associated with other medications.

  • ACE inhibitors like enalapril and benazepril
  • Aminosalicylates such as sulfasalazine and mesalamine
  • Other myelosuppressive drugs like trimethoprim/sulfa and cyclophosphamide
  • Some muscle relaxants
  • Warfarin (an anticoagulant drug)

Never change your dog's prescribed treatments without first speaking with your veterinarian.