Azathioprine Side Effects in Dogs

Azathioprine can cause lethargy.
Azathioprine can cause lethargy. (Image: Aly Tyler/iStock/Getty Images)

According to, azathioprine is an immunosuppressive drug that is chemically related to some anti-cancer chemotherapy agents. Azathioprine is used to suppress cells involved in autoimmune diseases, such as lupus. It interferes with the metabolism of immune and antibody-producing cells known as lymphocytes.

Common Side Effects

Azathioprine can cause lethargy, fever, reluctance to move, and loss of appetite. Common side effects like these affect all animals taking the drug, including dogs. Dogs tend to be more reluctant to move than other animals and tire more often on azathioprine.

Less Common Side Effects

Transient diarrhea or loose stools and vomiting, which can signal pancreatitis, are less common symptoms that can occur with the use of azathioprine. Dogs also have an increased risk of infection and neoplasia, which is the abnormal proliferation of cells.

Other Problems

Some dogs may be more likely to develop leukopenia (decrease in the white blood cells found in the blood), anemia, thrombocytopenia (presence of very few platelets in the blood), and hepatotoxicosis (accumulation of copper in the liver). Approximately 10 percent of dogs lack the proper enzymes necessary to metabolize azathioprine. All animals treated with this drug should receive regular blood tests to monitor numbers of blood cells.

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