Imuran reduces the strength of the immune system and is called an immunosuppressive. Imuran is used alone or in combination with other medicines including corticosteroids. Imuran is used to reduce the immune system response against an organ transplant. Imuran is also used to treat autoimmune diseases (including severe rheumatoid arthritis) and inflammatory bowel disease (ulcerative colitis or Chrohn's disease). Response to therapy may only be evident after several weeks or months.