Rejection is a common problem in organ transplant patients. They may experience an episode of acute rejection or chronic rejection weeks, months or years after a transplant. Rejection happens when the immune system recognizes the transplanted organ as non-self and attacks it. This can be prevented to a certain degree with the use of immunosuppressants, also known as anti-rejection medication. There are many types of immunosuppressants, such as azathioprine, cyclosporine, and prednisone. These medications work by suppressing the immune system and reduce its ability to reject the organ. They should be taken strictly according to the doctor's orders. Even one missed dose can put patients at risk of organ rejection. If patients experience organ rejection, doctors will change the type of medication, increase the dose or add another medication.