Azathioprine drug, molecular model. Azathioprine is used as an immunosuppressant. It works by inhibiting purine synthesis, which is essential for cell proliferation. It was used in cancer chemotherapy, but this use has dwindled as more powerful drugs have been developed. However it is still widely used to suppress the immune system, which is done after organ transplants to prevent rejection of the new organ, and also to treat autoimmune disorders such as rheumatoid arthritis. In 2005 concerns were raised that prolonged use of the drug raises the risk of developing skin cancer. In this molecule, carbon atoms are orange, nitrogen is yellow, sulphur blue, oxygen green and hydrogen white.