Tacrolimus is a medicine which can be used as a treatment for ulcerative proctitis or colitis that has proved to be resistant to, or needed multiple courses of, steroid therapy. It controls the body’s inflammatory response by targeting the immune system. It works in a similar way to azathioprine, and is usually given to people who have not got any better on azathioprine or mercaptopurine, or had side effects meaning they cannot take it. Tacrolimus has been used in high doses to help prevent rejection of organs in patients undergoing organ transplants, such as kidney transplants. It is normal to expect to be taking tacrolimus for a few months, and perhaps up to at least one year, although this will depend on your response to the tablets.