Opioids are substances that act on opioid receptors to produce morphine-like effects.^[1] Opioids are most often used medically to relieve pain,^[2] and by people addicted to opioids.^[3] Opioids include opiates, an older term that refers to such drugs derived from opium, including morphine itself.^[4] Other opioids are semi-synthetic and synthetic drugs such as hydrocodone, oxycodone and fentanyl; antagonist drugs such as naloxone and endogenous peptides such as the endorphins.^[5] The terms opiate and narcotic are sometimes encountered as synonyms for opioid. Opiate is properly limited to the natural alkaloids found in the resin of the opium poppy although some include semi-synthetic derivatives.^[4][6] Narcotic, derived from words meaning numbness or sleep, as an American legal term, refers to cocaine and opioids, and their source materials; it is also loosely applied to any illegal or controlled psychoactive drug.^[7][8] In some jurisdictions all controlled drugs are legally classified as narcotics. The term can have pejorative connotations and its use is generally discouraged where that is the case.^[9][10]

Primarily used for pain relief, including anesthesia they are also used to suppress cough, suppress diarrhea, treat addiction, reverse opioid overdose, and suppress opioid induced constipation.^[11] Extremely strong opioids are approved only for veterinary use such as immobilizing large mammals.^[12] Opioids act by binding to opioid receptors, which are found principally in the central and peripheral nervous system and the gastrointestinal tract. These receptors mediate both the psychoactive and the somatic effects of opioids. Opioid drugs include partial agonists, like the anti-diarrhea drug loperamide and antagonists like naloxegol for opioid-induced constipation, which do not cross the blood-brain barrier, but can displace other opioids from binding in those receptors.

The side effects of opioids may include itchiness, sedation, nausea, respiratory depression, constipation, and euphoria. Tolerance and dependence will develop with continuous use, requiring increasing doses and leading to a withdrawal syndrome upon abrupt discontinuation. The euphoria attracts recreational use, and frequent, escalating recreational use of opioids typically results in addiction. Accidental overdose or concurrent use with other depressant drugs commonly results in death from respiratory depression.^[13] Because of opioid drugs’ reputation for addiction and fatal overdose, most are controlled substances.

Illicit production, smuggling, and addiction to opioids prompted treaties, laws and policing which have realized limited success. In 2013 between 28 and 38 million people used opioids illicitly (0.6% to 0.8% of the global population between the ages of 15 and 65).^[14] In 2011 an estimated 4 million people in the United States used opioids recreationally or were dependent on them.^[15] Current increased rates of recreational use and addiction are attributed to over-prescription of opioid medications and inexpensive illicit heroin.^[16][17][18] Conversely, fears about over-prescribing, exaggerated side effects and addiction from opioids are similarly blamed for under-treatment of pain