Branch of medicine dealing with the actions of drugs in the bodyboth therapeutic and toxic effectsand 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. See alsodrug poisoning; pharmacy; pharmaceutical industry.