The use of chemicals (drugs or medications) to kill malignant (cancer) cells. Numerous chemicals have been developed for this purpose, and most act to injure the DNA of cells. When the DNA is injured, the cells cannot grow or survive. Successful chemotherapy depends on the malignant cells being at least somewhat more sensitive to the chemicals than the normal cells. Because the cells of the marrow, the intestinal tract, the skin, and hair follicles are most sensitive to these chemicals, injury to these organs are common side effects of chemotherapy, i.e., mouth sores, diarrhea, rashes, and hair loss.