This is common but by no means inevitable. It mostly depends on the specific drug being given. Chemotherapy is treatment by anti-cancer medicines given by injection into the blood stream or by pill. With certain limitations, it goes to all parts of the body and can treat cancer in any location. Almost all chemotherapeutic agents act by interfering with cell division.
Cancer is the result of uninhibited cell division.
If the cancer cells can be prevented from dividing then the cancer will not grow. Normal cells also must divide to replace those lost through wear and tear as well as to repair tissue injuries. These normal cells are also affected by the chemotherapy. Chemotherapy is also referred to as cyto-toxic therapy. All chemotherapeutic agents have side effects.