Azathioprine belongs to the group of medications known as immunosuppressive agents. It is used to prevent the rejection of kidney transplants and to treat severe rheumatoid arthritis that is not responding to conventional treatment. Azathioprine prevents rejection of kidney transplant and reduces the inflammation and pain in rheumatoid arthritis by suppressing the body's natural defense, or immune system. Your doctor may have suggested this medication for conditions other than the ones listed in these drug information