Chemotherapy uses cytotoxic (cell-killing) drugs that target rapidly dividing cells. Because it travels through the bloodstream, it can reach cancer that has spread — unlike surgery or radiation, which treat one area. Many regimens combine several drugs with different mechanisms (alkylating agents, antimetabolites, anthracyclines, platinum agents, taxanes and others) to hit cancer in multiple ways and limit resistance. It may be given before surgery to shrink a tumor (neoadjuvant), after surgery to kill remaining cells (adjuvant), together with radiation (chemoradiation), or as the main treatment for advanced disease. Side effects come from harm to normal fast-growing cells (bone marrow, hair, gut lining).
Most often by IV infusion (frequently through a port or central line), and sometimes as pills. Given in cycles — treatment days followed by rest to let normal cells recover — over months, usually as an outpatient. The exact drugs, doses and schedule depend on the cancer type and goal.
Side effects vary by drug and typically improve between cycles and after treatment finishes. Blood counts are monitored closely. Supportive medicines (anti-nausea drugs, growth factors) make modern chemo far more tolerable than in the past.