A drug, broadly speaking, is any substance that, when absorbed into the body of a living organism, alters normal bodily function. There is no single, precise definition, as there are different meanings in drug control law, government regulations, medicine, and colloquial usage."Drug." Dictionary.com Unabridged (v 1.1), Random House, Inc., via dictionary.com. Retrieved on 2007-09-20.

In pharmacology, Dictionary.com defines a drug as "a chemical substance used in the treatment, cure, prevention, or diagnosis of disease or used to otherwise enhance physical or mental well-being." Drugs may be prescribed for a limited duration, or on a regular basis for chronic disorders."Drug." The American Heritage Science Dictionary, Houghton Mifflin Company, via dictionary.com. Retrieved on 2007-09-20.

Recreational drugs are chemical substances that affect the central nervous system, such as opioids or hallucinogens. They may be used for perceived beneficial effects on perception, consciousness, personality, and behavior."Drug." Merriam-Webster's Medical Dictionary, Merriam-Webster, Inc., via dictionary.com. Retrieved on 2007-09-20. Some drugs can cause addiction and habituation.

Drugs are usually distinguished from endogenous biochemicals by being introduced from outside the organism.Fact: date=September 2007 For example, insulin is a hormone that is synthesized in the body; it is called a hormone when it is synthesized by the pancreas inside the body, but if it is introduced into the body from outside, it is called a drug.Fact: date=September 2007

Many natural substances such as beers, wines, and some mushrooms, blur the line between food and drugs, as when ingested they affect the functioning of both mind and body.

Medication

right main: pharmaceutical drug 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 (OTC) medications, which are available in pharmacies and supermarkets without special restrictions, behind-the-counter (BTC), which are dispensed by a pharmacist without needing a doctor's prescription, and Prescription only medicines (POM), which must be prescribed by a licensed medical professional, usually a physician.Fact: date=September 2007