Chemotherapy refers to the use of chemical agents to stop the growth of cancer cells, either by killing the cells or by stopping them from dividing. Most chemotherapeutic agents are "cytotoxic," meaning they are toxic to cells. Their primary mechanism of action involves interfering with the cell cycle, specifically targeting rapidly dividing cells. Because cancer cells typically divide more frequently than healthy cells, they are more susceptible to these agents.

Agents are generally classified into two categories: Cell-Cycle Specific (CCS) and Cell-Cycle Non-Specific (CCNS). CCS drugs, such as antimetabolites (e.g., Methotrexate) and plant alkaloids (e.g., Vincristine), are effective only when the cell is in a specific phase, such as the S-phase or M-phase. CCNS drugs, like Alkylating agents (e.g., Cyclophosphamide), can kill cells regardless of which phase they are in. For a detailed breakdown of the therapeutic classes and their molecular targets, the Chemotherapy Market documentation provides a technical overview. Understanding these cycles is critical for clinicians when designing "Dose-Dense" or "Metronomic" schedules to maximize tumor kill while allowing healthy tissue recovery.

The major challenge in chemotherapy is "Selectivity." Since the drugs cannot perfectly distinguish between malignant and healthy rapidly dividing cells (like those in the bone marrow, GI tract, and hair follicles), patients often experience systemic side effects. Research is currently focused on "Targeted Chemotherapy," where cytotoxic drugs are conjugated to antibodies (ADCs) to deliver the "payload" directly to the tumor site, minimizing collateral damage.