Azathioprine (AZA), also known by its brand name Imuran, is an immunosuppressive drug used in organ transplantation and autoimmune diseases and belongs to the chemical class of purine analogues. Synthesized originally as a cancer drug and a prodrug for mercaptopurine in 1957, it has been widely used as an immunosuppressant for more than 50 years. Azathioprine is produced by a number of manufacturers under different brand names. It is on the World Health Organization's List of Essential Medicines, a list of the most important medication needed in a basic health system.