chemotherapy (kē´mōthĕr´əpē), treatment of disease with chemicals or drugs. One chemotherapeutic approach is the development of selectively toxic substances, i.e., substances that can destroy or inhibit infecting organisms or, as in cancer, malignant tissue, but do not damage normal host tissue. In treating infection, selectively toxic agents may block a biochemical reaction necessary to the viability of the pathogen but not to that of the host; for example, penicillin blocks synthesis of bacterial cell walls, a component animal cells lack. Other chemotherapeutic substances differentially affect biochemical reactions in different tissues; thus antimetabolites such as methotrexate and Cytoxan are more toxic to rapidly proliferating cells such as those associated with cancer than to normal cells. Other drugs act in various ways to produce effects that initiate or enhance some normal body function; for instance, neostigmine blocks the action of an enzyme limiting transmission of nerve impulses and thereby acts as a nervous system stimulant. The usefulness of chemotherapeutic agents also depends on their pharmacological action, e.g., their rate of absorption, rapidity of action and rate of excretion, degree of storage in the body, effects of products of their metabolic breakdown, and potential for causing hypersensitivity reactions. Some drugs are given prophylactically, to prevent infection, e.g., penicillin is given to rheumatic fever patients to prevent reinfection by the causative organism, the streptococcal bacterium.