Chemo brain - side effects of cancer therapy What is cancer chemotherapy? Cancer chemotherapy is a type of cancer treatment. It uses medicines to destroy cancer cells. Normally, the cells in your body grow and die in a controlled way. Cancer cells keep growing without control. Chemotherapy works by killing the cancer cells, stopping them from spreading, or slowing their growth. Chemotherapy is used to: Treat cancer by curing the cancer, lessening the chance it will return, or stopping or slowing