Azathioprine is the generic form of the brand name drug Imuran, which is used along with other medicines to prevent organ rejection after a kidney transplant.The drug is also used to treat severe rheumatoid arthritis, ulcerative colitis, and Crohn's disease.Azathioprine is in class of drugs known as immunosuppressants. It works by altering the activity of the body's immune system so it won't attack a transplanted organ or joint.zathioprine is not approved by the FDA to treat Crohn's or ulcerative colitis, but some doctors may prescribe the medicine off-label. You should call your doctor immediately if you are taking the drug for Crohn's or ulcerative colitis and experience the following symptoms: 1  Fever.