What is Azathioprine?

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  • Written By: A. Gabrenas
  • Edited By: Jacob Harkins
  • Last Modified Date: 17 January 2020
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Azathioprine is a medication that suppresses the immune system. It may be used to help treat severe rheumatoid arthritis, to prevent the body’s rejection of a transplanted kidney or for other conditions related to immune responses. Healthcare providers often warn of many possible risks and side effects associated with the use of azathioprine, frequently due to its action of immune suppression. Due to the drug’s potent action, healthcare providers sometimes recommend against using this treatment in people with certain health conditions.

The exact mechanism by which azathioprine works is not well understood by most experts. It is believed, however, that the drug makes the cells of the immune system less sensitive, which in turns generally makes them less likely to attack perceived invaders. This is why it is also known as an immunosuppressant.

In the case of kidney transplantation, the body’s immune system normally attacks the new kidney because it does not recognize it as its own. For rheumatoid arthritis, the immune system gets inappropriately activated and begins to attack the body’s own joints. Azathioprine can help slow or stop these attacks. Some healthcare providers may also prescribe it to help treat other illnesses related to the immune system, such as ulcerative colitis and lupus, both of which are autoimmune disorders, similar to rheumatoid arthritis. The drug isn’t a cure for any of these conditions.


For most conditions, azathioprine is usually given as a tablet. There is a form available that may be injected, but this route of administration is less common. The dosage prescribed is usually calculated based on the patient’s weight. Due to the fact that azathioprine suppresses the immune system, it has the potential to have a host of negative side effects. One of the most serious of these is the increased risk of cancer, specifically lymphoma and skin cancer. The risk of this can be increased in patients who have taken certain other medications, such as chlorambucil.

Another serious possible side effect is a decrease in the number of white blood cells and platelets in the body. This may lead to an increased risk of infection, bruising and bleeding. Patients taking the medication are often given regular blood tests to check his or her blood cell counts. Over time, as the drug is eliminated from the body, the blood cell counts typically return to normal.

Healthcare providers generally warn against using azathioprine in certain patients. This includes patients who have infections, since suppressing the immune system further can make the infections worse. It is also generally contraindicated in women who are pregnant or planning to become pregnant, because it has been shown to cause birth defects in animal studies. Patients who lack a gene to produce an enzyme known as TPMT, which helps break down the medication in the body, are also generally advised not to take the drug due to the risk of toxicity. Testing to see if patients have the TPMT gene may be done before treatment is begun.



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