<P> A drug is any substance (other than food that provides nutritional support) that, when inhaled, injected, smoked, consumed, absorbed via a patch on the skin, or dissolved under the tongue causes a temporary physiological (and often psychological) change in the body . </P> <P> In pharmacology, a drug is a chemical substance of known structure, other than a nutrient of an essential dietary ingredient, which, when administered to a living organism, produces a biological effect . A pharmaceutical drug, also called a medication or medicine, is a chemical substance used to treat, cure, prevent, or diagnose a disease or to promote well - being . Traditionally drugs were obtained through extraction from medicinal plants, but more recently also by organic synthesis . Pharmaceutical drugs may be used for a limited duration, or on a regular basis for chronic disorders . </P> <P> Pharmaceutical drugs are often classified into drug classes--groups of related drugs that have similar chemical structures, the same mechanism of action (binding to the same biological target), a related mode of action, and that are used to treat the same disease . The Anatomical Therapeutic Chemical Classification System (ATC), the most widely used drug classification system, assigns drugs a unique ATC code, which is an alphanumeric code that assigns it to specific drug classes within the ATC system . Another major classification system is the Biopharmaceutics Classification System . This classifies drugs according to their solubility and permeability or absorption properties . </P> <P> Psychoactive drugs are chemical substances that affect the function of the central nervous system, altering perception, mood or consciousness . They include alcohol, a depressant (and a stimulant in small quantities), and the stimulants nicotine and caffeine . These three are the most widely consumed psychoactive drugs worldwide and are also considered recreational drugs since they are used for pleasure rather than medicinal purposes . Other recreational drugs include hallucinogens, opiates and amphetamines and some of these are also used in spiritual or religious settings . Some drugs can cause addiction and all drugs can have side effects . Excessive use of stimulants can promote stimulant psychosis . Many recreational drugs are illicit and international treaties such as the Single Convention on Narcotic Drugs exist for the purpose of their prohibition . </P>

What is the definition of the word drug