Opioids are substances that act on opioid receptors to produce morphine-like effects. Medically they are primarily used for pain relief, including anesthesia. Other medical uses include suppression of diarrhea, treating opioid use disorder, reversing opioid overdose, suppressing cough, and suppressing opioid-induced constipation. Extremely potent opioids such as carfentanil are only approved for veterinary use. Opioids are also frequently used non-medically for their euphoric effects or to prevent withdrawal.

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. An overdose or concurrent use with other depressant drugs commonly results in death from respiratory depression.

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.

Because of opioid drugs’ reputation for addiction and fatal overdose, most are controlled substances. 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). In 2011, an estimated 4 million people in the United States used opioids recreationally or were dependent on them. As of 2015, increased rates of recreational use and addiction are attributed to over-prescription of opioid medications and inexpensive illicit heroin. Conversely, fears about over-prescribing, exaggerated side effects and addiction from opioids are similarly blamed for under-treatment of pain

Global estimates of illegal drug users in 2014
(in millions of users)
Substance Best
estimate Low
estimate High
type stimulants 35.65 15.34 55.90
Cannabis 182.50 127.54 233.65
Cocaine 18.26 14.88 22.08
Ecstasy 19.40 9.89 29.01
Opiates 17.44 13.74 21.59
Opioids 33.12 28.57 38.52
Opioid prescriptions in the US increased from 76 million in 1991 to 207 million in 2013.

In the 1990s, opioid prescribing increased significantly. Once used almost exclusively for the treatment of acute pain or pain due to cancer, opioids are now prescribed liberally for people experiencing chronic pain. This has been accompanied by rising rates of accidental addiction and accidental overdoses leading to death. According to the International Narcotics Control Board, the United States and Canada lead the per capita consumption of prescription opioids. The number of opioid prescriptions in the United States and Canada is double the consumption in the European Union, Australia, and New Zealand. Certain populations have been affected by the opioid addiction crisis more than others, including First World communities and low-income populations. Public health specialists say that this may result from unavailability or high cost of alternative methods for addressing chronic pain