Imatinib ( i-MAT-in-ib) is a new type of medication that prevents and stops the growth of cancer cells. It helps your body fight against a type of cancer called chronic myeloid leukemia (CML) or gastrointestinal stromal tumor (GIST). CML is a disease in which your body makes too many abnormal white blood cells which can cause you to become sick more often and also to feel weak or tired. Imatinib helps your body to stop making these abnormal white blood cells. GIST is a group of cancer cells that started growing in the wall of the stomach, intestines, or rectum. Imatinib helps your body to stop making these abnormal cells.