Chemotherapy, commonly shortened to "chemo," refers to the medical use of chemical agents to treat disease. In modern clinical practice the term most often denotes systemic drug treatment for cancer, using medicines designed to kill or slow the growth of rapidly dividing cells. These agents are frequently described as cytotoxic because they can damage or destroy living cells.

How chemotherapy works

Chemotherapy drugs interfere with cellular processes that are essential for growth and replication. Different drug classes target distinct stages of the cell cycle or specific molecular pathways. Because cancer cells typically divide faster than most normal cells, they are more vulnerable to these agents; however, normal tissues with rapid turnover (for example, bone marrow, hair follicles and the lining of the gut) are also affected. Clinicians select agents based on the biological features of the disease and the intended treatment goal.

Types and schedules

There is no single chemotherapy regimen. Drugs may be used alone or combined to increase effectiveness and to reduce the chance of resistance. Common ways chemotherapy is delivered include:

  • Systemic administration (intravenous or oral) to reach disease throughout the body.
  • Regional delivery to concentrate effect in a particular area.
  • Short courses before surgery (neoadjuvant) to shrink tumors, or after surgery (adjuvant) to reduce recurrence risk.

Physicians balance dose intensity and timing to maximize benefit while managing toxicity. Choice of regimen depends on tumor type, stage, patient health and treatment goals.

Uses beyond cancer

Although chemotherapy is most commonly associated with oncology, some cytotoxic and immunomodulatory agents are used for nonmalignant diseases. Examples include severe autoimmune disorders where these medicines suppress pathological immune activity. A specific example is the use of certain drugs in rheumatoid arthritis and other inflammatory conditions. In these settings the name "chemotherapy" may be less frequently used, but the principle—using chemicals to alter cellular or immune behavior—remains similar.

Side effects and management

Chemotherapy can cause a range of adverse effects because it impacts normal as well as abnormal cells. Typical problems include hair loss, fatigue, nausea, reduced blood counts and increased infection risk. Modern supportive care—anti-nausea medications, growth factors, transfusions, and dose adjustments—aims to reduce these harms and allow patients to complete effective treatment.

  • Common side effects: nausea, mucositis, hair thinning, bone marrow suppression.
  • Supportive strategies: symptom-directed drugs, infection prevention and close monitoring.

History, outcomes and notable points

The development of systemic chemotherapy began in the mid-20th century and expanded as new drug classes and combinations were discovered. Over decades, many cancers that were once uniformly fatal have become curable or manageable chronic diseases thanks in part to chemotherapy. Treatment remains individualized: different cancers and patients require tailored approaches, and ongoing research seeks more selective agents with fewer off-target effects. For introductory resources on how chemotherapeutic agents are classified and chosen, see general information on chemical substances and disease management, as well as topics related to cancer.