Many breast cancer patients will fight their cancer with chemotherapy — potent cancer cell-killing drugs that are ingested orally or injected intravenously. When prescribed, chemotherapy is likely necessary but patients should be made aware of side effects that could occur during or after treatment. Here are eight facts about chemotherapy side effects according to the American Cancer Society: 1. Chemotherapy causes side effects because, in addition to killing off cancer cells, the drugs will also target healthy cells. Fast-growing cells such as those in hair follicles, the mouth, digestive tract, reproductive organs, and bone marrow cells are particularly impacted.