Chemotherapy (often called just "chemo") is treatment with cancer-killing drugs that may be given intravenously (injected into a vein) or taken as a pill or a liquid. The drugs travel through the bloodstream to reach cancer cells in most parts of the body, making this treatment useful for cancers that have spread to distant organs. Chemo can also be used to kill any cancer cells that may have been left behind after surgery or to shrink tumors before surgery. Chemo is given in cycles, with each period of treatment followed by a recovery period. Treatment usually lasts for several months. While these drugs kill cancer cells, they also damage some normal cells, which can lead to side effects like hair loss, mouth sores, and nausea.
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American Cancer Society answered
Chemotherapy is the use of anti-cancer drugs to stop tumor growth and kill cancer cells in the body. Chemotherapy can also be used to shrink the tumor, or used after surgery to help reduce recurrence from cancer cells that may remain after surgery. The type of chemotherapy treatment acts on cancer cells differently; your medical oncologist will determine the best type of chemotherapy for your specific disease state.
Chemotherapy is systemic drug therapy given either in pills or intravenously. By interfering with the ability of cancer cells to split and grow in the body, chemotherapy can help to shrink the size of existing tumors and can help prevent recurrences in patients who have had cancer. While surgery and radiation treat any cancer cells specifically in the breast or underarm, chemotherapy treats cancer cells that may be present throughout the entire body.