pharmacology, study of the changes produced in living animals by chemical substances, especially the actions of drugs, substances used to treat disease. Systematic investigation of the effects of drugs based on animal experimentation and the use of isolated and purified active substances developed in the mid-19th cent. Pharmacologists, emphasizing the mechanisms by which drugs act, draw on the disciplines of physiology, pathology, biochemistry, and bacteriology. Pharmacology embraces a number of sciences, including pharmacodynamics (the study of the action of drugs on a living body), therapeutics (use of drugs and method of administration in treatment for disease), materia medica (study of the source, composition, characteristics, and preparation of drugs), toxicology (the study of poisons and their action and of methods of treating poisoning), pharmaceutical chemistry (chemistry in relation to drugs), and pharmacy (the preparation and dispensing of drugs for medical use).

Branch of medicine dealing with the actions of drugs in the body—both therapeutic and toxic effects—and development and testing of new drugs and new uses of existing ones. Though the first Western pharmacological treatise (a listing of herbal plants) was compiled in the 1st century AD, scientific pharmacology was possible only from the 18th century on, when drugs could be purified and standardized. Pharmacologists develop drugs from plant and animal sources and create synthetic versions of these, along with new drugs based on them or their chemical structure. They also test drugs, first in vitro (in the laboratory) for biochemical activity and then in vivo (on animals, human volunteers, and patients) for safety, effectiveness, side effects, and interactions with other drugs and to find the best dose, timing, and route (mouth, injection, etc.). Drug products are constantly tested for potency and purity. Seealso drug poisoning; pharmacy; pharmaceutical industry.

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