DRUG

any chemical agent that affects the functions of living things. Drugs are used in treating, diagnosing, and preventing disease. Commonly used types of drugs include antibiotics, stimulants, tranquilizers, sedatives and hypnotics, antidepressants, analgesics, narcotics, anesthetics, hormones, and a wide variety of preparations for specific purposes, such as laxatives, heart stimulants, anti-blood-clotting agents, diuretics, and antihistamines. Vaccines, which are preparations of killed or weakened bacteria or viruses used to stimulate resistance to subsequent infections, are sometimes also considered drugs. Vaccination has proved particularly effective in preventing viral diseases. While bacterial infections have largely succumbed to antibiotics and other drugs, viral diseases still remain largely impossible to treat with drugs because it is difficult to find chemical agents that can selectively destroy viral nucleic acids or enzymes without harming those of the infected cells. Drugs employed in cancer therapy are among the more recent advances in drug research. Every drug that has been developed to kill cancer cells, however, also affects rapidly dividing normal cells. Because each drug exerts its effects at different phases of the cell cycle, complex regimens of anticancer drugs may be used to reduce the incidence of cellular resistance and side effects. Drugs can be given by enteral or parenteral administration. Enteral administration involves the alimentary tract and includes such methods as oral and rectal administration. Parenteral methods encompass all other routes, including injection, inhalation, and percutaneous administration (absorption through the skin). Drugs are able to affect the functioning of living things by interrupting the extensive chemical system of communications that regulates the integrated functioning of the cells of the organism. The effects of drugs mimic those of endogenous substances of the body, and thus they do not impart new activities to the cells. In order for a drug molecule of a certain molecular configuration to be effective, it must bind with a specific, complementary site that is often located on the membrane of the cell and that is called a receptor. The drugreceptor complex thus formed triggers certain biochemical changes within the cell to bring about specific biochemical effects. Drugs that bind to receptors and actively bring about biochemical changes are called agonists. Drugs that compete with agonists (which can be other drugs or endogenous molecules) but do not in themselves bring about an effect are called antagonists. Regardless of whether a drug is an agonist or an antagonist, it will not exert its influence on target molecules or cells if the receptor configuration is not complementary. The oldest existing catalogue of drugs (called pharmacopoeia) is a stone tablet from ancient Babylonia (c. 1700 BC). The development of modern pharmacology began in the 19th century, when chemicals with drug action were first isolated from plants containing them. Drug research made another leap forward with the discovery of antibiotics in 1928. Most drugs today are products of chemical synthesis, but numerous drugs of plant, animal, mineral, and microbial origin are still important. Recently, genetic engineering has provided a new source of compounds that are too complex to synthesize economically. any chemical substance that affects the functioning of living things and the organisms (such as bacteria, fungi, and protozoans) that infect them. Pharmacology, the science of drugs, deals with all aspects of drugs in medicine, including their mechanism of action, physical and chemical properties, metabolism, and therapeutics and prophylaxis. Until the mid-19th century the approach to drug therapeutics was entirely empirical. This thinking changed when the mechanism of drug action began to be analyzed in physiological terms and when some of the first chemical analyses of naturally occurring drugs were performed. The end of the 19th century signaled the growth of the pharmaceutical industry and the production of the first synthetic drugs. Chemical synthesis has become the most important source of therapeutic drugs, although genetic engineering is developing as a means of synthesizing proteins. Drugs produce harmful as well as beneficial effects, and decisions about when and how to use them therapeutically always involve the balancing of benefits and risks. Drugs approved for human use are divided into ethical preparations, available only with a prescription, and over-the-counter drugs, which can be bought freely. The availability of drugs for medical use is regulated by law. Details of all approved substances are published in The United States Dispensatory and The National Formulary. Similar publications exist in other developed countries. Drug treatment is the most frequently used type of therapeutic intervention in medicine. Its power and versatility derive from the fact that the body relies extensively on chemical communications systems to achieve integrated function among billions of separate cells. The body is, therefore, highly susceptible to the calculated chemical subversion of parts of this communications network that occurs when drugs are administered.