Azathioprene /azathioprine (Imuran, Azasan), is a prodrug used to suppress the immune system, and therefore belongs to a class of medications called Immunosuppressants. A prodrug is a precursor to a drug, and is converted in the body to its active form: azathioprine is converted to mercaptopurine (purinethol) in the body. Although the exact mechanism of action of azathioprine is not known, it works to stop the proliferation of certain types of white blood cells called T and B lymphocytes that make up part of the body’s immune system. Consequently, it is used to treat autoimmune disorders including skin diseases like eczema—or atopic dermatitis. It is also FDA approved for use in organ transplant patients to prevent organ rejection, and active rheumatoid arthritis. Azathioprine is available by prescription only, and comes in 50mg, 75mg and 100mg tablets as well as 10mg injections. It has an FDA pregnancy category rating of D, which means that it does cross the placenta and can be found in cord blood. Azathioprine carries a warning which states that it can cause a decrease in the number of blood cells in your bone marrow, and may increase the risk for certain types of cancer including skin cancer and lymphoma.