Azathioprine is used to prevent organ rejection in people who have received a kidney transplant. It is usually taken along with other medications to allow your new kidney to function normally. Azathioprine is also used to treat rheumatoid arthritis. In this condition, the body’s defense system (immune system) attacks healthy joints. Azathioprine belongs to a class of drugs known as immunosuppressants. It works by weakening the immune system to help your body accept the new kidney as if it were your own (in the case of an organ transplant) or to prevent further damage to your joints (in the case of rheumatoid arthritis).