A drug, broadly speaking, is any chemical substance that, when absorbed into the body of a living organism, alters normal bodily function.^[5] There is no single, precise definition, as there are different meanings in medicine, government regulations, and colloquial usage.^[6]

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."^[6] Drugs may be prescribed for a limited duration, or on a regular basis for chronic disorders.^[7]

Recreational drugs are chemical substances that affect the central nervous system, such as opioids or hallucinogens.^[7] They may be used for perceived beneficial effects on perception, consciousness, personality, and behavior.^[7][8] Some drugs can cause addiction and habituation