A drug is, in the broadest of terms, a chemical substance that has known biological effects on humans or other animals. Foods are generally excluded from this definition, in spite of their physiological effects on animal species.
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."
Recreational drugs are chemical substances that affect the central nervous system, such as opioids or hallucinogens.
They may be used for effects on perception, consciousness, personality, and behavior. Many recreational drugs are also used in medicine. 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.
Etymology
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" arose later and invokes the psychoactive rather than medicinal properties of a substance.
Medication
A medication or medicine is a drug taken to cure and/or ameliorate any symptoms of an illness or medical condition, or may be used as preventive medicine that has future benefits but does not treat any existing or pre-existing diseases or symptoms.
Dispensing of medication is often regulated by governments into three categories—over-the-counter medications, which are available in pharmacies and supermarkets without special restrictions, behind-the-counter, which are dispensed by a pharmacist without needing a doctor's prescription, and prescription only medicines, which must be prescribed by a licensed medical professional, usually a physician.
In the United Kingdom, BTC medicines are called pharmacy medicines which can only be sold in registered pharmacies, by or under...