A drug, in the broadest term, is a chemical substance that, when ingested, has a high biological response to quantity ratio compared to regular foods. Regular foods require much greater quantities to achieve biological effect and are generally excluded from the popular definition.

In pharmacology, a drug is "a chemical substance used in the treatment, cure, prevention, or diagnosis of disease or used to otherwise enhance physical or mental well-being." Traditionally drugs were gained through extraction from different natural sources (e.g., 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. Pharmaceutical drugs are often classified into groups of related drugs known as a drug class, which have similar chemical structures, the same mechanism of action (i.e., bind to the same biological target), a related mode of action, and/or 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.

Psychoactive drugs are chemical substances that affect the function of the nervous system, altering perception, mood or consciousness. Alcohol, nicotine, and caffeine are the most widely consumed psychoactive drugs worldwide. Recreational drugs are drugs that are not used for medicinal purposes, but are instead used for pleasure. Common recreational drugs include alcohol, nicotine and caffeine, as well as other substances such as opiates and amphetamines. Some drugs can cause addiction and habituation and all drugs can cause side effects. Many drugs are illegal for recreational purposes and international treaties such as the Single Convention on Narcotic Drugs exist for the purpose of legally prohibiting certain substances.


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