Azathioprine is an immunosuppressant medicine, which means it weakens your body’s defence (immune) response. It belongs to a class of medicines known as disease-modifying antirheumatic drugs (DMARDs).
It is used to treat a number of autoimmune conditions related to an ‘overactive’ immune system, such as Crohn's disease, ulcerative colitis and rheumatoid arthritis. Autoimmune means that your body ‘attacks’ itself, without any real known cause or reason.
Azathioprine is also used to stop transplant rejection for people who have had transplant surgery.