drug is any substance (other than that provides nutritional support) that, when food , inhaled , injected , smoked , consumed via a absorbed on the skin, or patch causes a physiological (and often psychological) change in the body. dissolved under the tongue  
, 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. pharmacology A  , also called a medication or medicine, is a chemical substance used to pharmaceutical drug , treat , cure , or prevent a diagnose or to promote disease . well-being Traditionally drugs were obtained through extraction from  , but more recently also by medicinal plants . organic synthesis Pharmaceutical drugs may be used for a limited duration, or on a regular basis for  . chronic disorders 
Pharmaceutical drugs are often classified into
—groups of related drugs that have similar drug classes , the same chemical structures (binding to the same mechanism of action ), a related biological target , and that are used to treat the same disease. mode of action  The  (ATC), the most widely used drug classification system, assigns drugs a unique Anatomical Therapeutic Chemical Classification System , which is an alphanumeric code that assigns it to specific drug classes within the ATC system. Another major classification system is the ATC code . This classifies drugs according to their solubility and permeability or Biopharmaceutics Classification System properties. absorption  are chemical substances that affect the function of the Psychoactive drugs , altering central nervous system , perception or mood . consciousness They include  , a alcohol (and a stimulant in small quantities), and the depressant stimulants and nicotine . These three are the most widely consumed psychoactive drugs worldwide caffeine and are also considered  since they are used for pleasure rather than medicinal purposes. recreational drugs Other recreational drugs include  , hallucinogens and opiates and some of these are also used in spiritual or religious settings. Some drugs can cause amphetamines addiction and all drugs can have  . side effects Excessive use of stimulants can promote  . Many recreational drugs are stimulant psychosis and international treaties such as the illicit exist for the purpose of their Single Convention on Narcotic Drugs . prohibition
In English, the noun "drug" is thought to originate from Old French "drogue", possibly deriving later into "droge-vate" from Middle Dutch meaning "dry barrels", referring to medicinal plants preserved in them.
The transitive verb "to drug" (meaning intentionally administer a substance to someone, often without their knowledge) arose later and invokes the psychoactive rather than medicinal properties of a substance. 
srpskohrvatski / српскохрватски: