Chemotherapy is a drug treatment that uses powerful chemicals to kill fast-growing cells in your body.Chemotherapy is most often used to treat cancer, since cancer cells grow and multiply much more quickly than most cells in the body.Many different chemotherapy drugs are available. Chemotherapy drugs can be used alone or in combination to treat a wide variety of cancers.Though chemotherapy is an effective way to treat many types of cancer, chemotherapy treatment also carries a risk of side effects. Some chemotherapy side effects are mild and treatable, while others can cause serious complications.