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An opioid is any drug that activates the opioid receptors found in the brain, spinal cord and gut. There are three broad classes of opioids: naturally occurring opium alkaloids, such as morphine (the prototypical opioid) and codeine; semi-synthetics such as heroin, oxycodone and hydrocodone that are produced by modifying natural opium alkaloids and have similar chemical structures; and pure synthetics such as fentanyl and methadone that are not produced from opium and may have very different chemical structures than the opium alkaloids.

Although the term opiate is often used to refer to all opium-like drugs, it is more properly limited to the natural opium alkaloids and the semi-synthetics derived from them.

There are at least three major classes of opioid receptors: μ, κ and δ. The μ receptor (the μ represents morphine) is the most important. Activation of the μ receptor by an agonist such as morphine causes analgesia, sedation, reduced blood pressure, itching, nausea, euphoria, decreased respiration, miosis (constricted pupils) and decreased bowel motility often leading to constipation. Some of these effects, such as sedation, euphoria and decreased respiration, tend to disappear with continued use as tolerance develops. Analgesia, miosis and reduced bowel motility tend to persist; little tolerance develops to these effects. Although tolerance to respiratory depression develops relatively quickly, it is the single most adverse side effect of opioid use; it is how overdoses kill. Opioid overdoses can be rapidly reversed with any of several opioid antagonists, drugs that bind to the μ receptors more strongly than most agonists but do not activate them. This displaces the agonist drug, countering its effects.

Opioids are widely used in medicine as analgesics (pain relievers); despite extensive research, no more effective analgesics have been found for severe pain. Extremely high doses are regularly used to treat the severe, chronic pain of terminal cancer; there is no upper limit to the dosage and the achievable pain relief as long as it is increased gradually to allow side effects (especially respiratory depression) to fade.

Opioids are also widely abused for their euphoria-producing properties. This is frequently problematic for the abuser because tolerance rapidly develops to this effect, and rapid dose escalations are required. A relatively small dose of a fast-acting opioid may produce intense euphoria in an "opioid naive" person, but once tolerance develops even very large doses may produce none at all.

Regular use of an opioid for any reason (pain relief, euphoria) rapidly induces physical dependence, characterized by a highly unpleasant withdrawal syndrome when the drug is discontinued or rapidly decreased in dosage, or an antagonist is administered. The acute withdrawal syndrome generally consists of signs and symptoms opposite to those of the drug when initially administered: severe dysphoria, anxiety, eye tearing, a runny nose, goosebumps, cramps and deep pains are common. The speed and severity of withdrawal depends on the half-life of the opioid; heroin withdrawal occurs more quickly and is more severe than methadone withdrawal, but methadone withdrawal takes longer. The acute withdrawal phase is often followed by a protracted phase of depression and insomnia that can last for months.

Physical dependence is completely distinct from (and does not imply) psychological addiction, defined as uncontrolled drug use despite harm. However, physical dependence can certainly aggravate psychological addiction when it occurs.

Withdrawal symptoms can be greatly lessened by slowly tapering the dose over days or weeks, sometimes after switching to a long-acting opioid such as methadone. The symptoms of opioid withdrawal can also be treated with other medications, such as clonidine for sympathetic hyperactivity and a benzodiazepine for anxiety and insomnia. "Rapid detox" is a relatively new technique that uses opioid antagonists to cause acute withdrawal while the patient is under general anesthesia to eliminate the otherwise extreme discomfort. This procedure has attracted considerable controversy due to its high cost and risk; several patients have died during the procedure. Many pain specialists consider the procedure unnecessary, and addiction specialists criticize it for doing nothing to keep an addict from relapsing into opioid abuse after the procedure is complete. Rapid detox also does not alleviate the protracted withdrawal syndrome that lasts for weeks or months after the acute phase.

Although physical dependence is nearly universal among those who use opioids regularly, true addiction is actually quite rare even when large amounts of opioids are used over long periods of time to treat chronic pain under the close supervision of a doctor. This is thought to be due to the rapid development of tolerance to the euphorigenic properties of opioids; without euphoria, only the unpleasant side effects (such as bowel dysfunction) remain so there is no motivation to take more than is needed to manage pain.

While opioids have long been used without controversy to treat acute pain (such as during recovery from surgery) and the pain of terminal cancer, in recent years they have seen greatly increased use in the management of chronic, non-malignant pain (i.e., pain with causes other than cancer, and where the patient has a long life expectancy). While this trend is still somewhat controversial in some circles, the emerging medical consensus is that most chronic pain patients can safely use opioids for years with a minimal risk of addiction or physical toxicity and an overall increase in quality of life and ability to function.

Phenanthrenes naturally occurring in opium:

Semisynthetic derivatives:

Fully synthetic:


Phenylpiperidines: diphenylpropylamine derivatives benzomorphan derivatives oripavine derivatives morphinan derivatives other opioids opioid antagonists (blockers)

Certain opioids, such as loperamide, cannot cross the blood-brain barrier. This makes them useful in the treatment of diarrhea, in that they exert their constipating effects on the gut without reaching the central nervous system.

See also

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