The prevention or treatment of disease by chemical substances. The term is generally used in two senses: the use of antibacterial and other drugs to treat infections; and the administration of ANTIMETABOLITES and other drugs to treat cancer. The discovery by Paul Ehrlich in 1910 of the action of Salvarsan in treating syphilis led to the introduction of sulphonamides in 1935, followed by PENICILLIN, which revolutionised the treatment of common infections during WWII. Many ANTIBACTERIAL drugs have been developed since then: these include CEPHALOSPORINS, cephamycins, TETRACYCLINES, AMINOGLYCOSIDES, MACROLIDES and CLINDAMYCIN as well as antituberculous drugs such as STREPTOMYCIN and METRONIDAZOLE. Unfortunately, overuse of chemotherapeutic drugs in medicine and in animal husbandry has stimulated widespread resistance among previously susceptible pathogenic micro-organisms. Chemotherapy also plays an important role in treating tropical diseases, especially MALARIA, SLEEPING SICKNESS and LEPROSY.

Chemotherapy is used widely in the treatment of cancer. Numerous drugs, generally CYTOTOXIC, are available; great care is required in their selection and to minimise side-effects. Certain tumours are highly sensitive to chemotherapy – especially testicular tumours, LEUKAEMIAS, LYMPHOMAS and various tumours occurring in childhood (e.g. Wilms’ tumour – see NEPHROBLASTOMA) – and may even be cured. (See also ANTIMETABOLITES.)