Any chemical substance that alters the physiological state of a living organism. Drugs are widely used in medicine for the prevention, diagnosis, and treatment of diseases; they include analgesics, antibiotics, anaesthetics, antihistamines, and anticoagulants. Some drugs are taken solely for the pleasurable effects they induce; these include narcotics; stimulants, such as cocaine and amphetamine; hallucinogens, such as LSD; and some tranquillizers. Many of these drugs are habit-forming and their use is illegal.