Azathioprine is a purine analog that is used as an immunosuppressive drug. Azathioprine was first introduced into clinical practice in 1960 and was replaced with cyclosporine in 1978 (Calne 1999). It is approved for the prevention of rejection in renal homotransplantation and the treatment of autoimmune diseases such as rheumatoid arthritis and inflammatory bowel disease. Azathioprine has been proposed for the treatment of multiple sclerosis and used in some European countries, but it has not been approved by the Food and Drug Administration for this indication in the United States. Other neurologic disorders where azathioprine has been used include neuromyelitis optica and myasthenia gravis.