Chemotherapy: A medical treatment modality, commonly used against cancer, by which strong chemicals are used to destroy rapidly dividing cells. One of the classic characteristics of cancer cells is that they divide uncontrollably: they can thus be killed by agents that target fast-growing cells. Unfortunately, chemotherapeutic drugs also, by their very nature, target healthy cells which divide rapidly: cells in our hair follicles, gastrointestinal tract, and bone marrow.

Chemotherapy is not necessarily used as the sole treatment against cancer, but can also be harnessed to shrink a tumour prior to or to kill any residual tumour cells following either surgery or radiotherapy. These potent drugs are also used in the context of bone marrow transplants and in lower doses to treat autoimmune diseases, such as rheumatoid arthritis.