Chemotherapy is the use of drugs to kill cancer cells. The doctor may use one drug or a combination of drugs. Chemotherapy may be the only kind of treatment a patient needs, or it may be combined with other forms of treatment. Most anti-cancer drugs are given by injection into a vein (IV); some are injected into a muscle or under the skin; and some are given by mouth. The side effects of chemotherapy depend mainly on the drugs and the doses the patient receives.