Opioid: 1. A synthetic narcotic that resembles the naturally occurring opiates. 2. Any substance that binds to or otherwise affects the opiate receptors on the surface of the cell.

The generic term applied to alkaloids from the opium poppy (Papaver somniferum), their synthetic analogues, and compounds synthesized in the body, which interact with the same specific receptors in the brain, have the capacity to relieve pain, and produce a sense of well-being (euphoria). The opium alkaloids and their synthetic analogues also cause stupor, coma, and respiratory depression in high doses.

Opium alkaloids and their semi-synthetic derivatives include morphine, diacetylmorphine (diamorphine, heroin), hydromorphine, codeine, and oxycodone. Synthetic opioids include levorphanol, propoxyphene, fentanyl, metha- done, pethidine (meperidine) and the agonist-antagonist pentazocine.

Endogenously occurring compounds with opioid actions include the endorphins and enkephalins (see opioid, endogenous). The most commonly used opioids (such as morphine, heroin, hydromorphine, methadone, and pethidine) bind preferentially to the .u-receptors; they produce analgesia, mood changes (such as euphoria, which may change to apathy or dysphoria), respiratory depression, drowsiness, psychomotor retardation, slurred speech, impaired concentration or memory, and impaired judgement.

Over time, morphine and its analogues induce tolerance and neuroadaptive changes that are responsible for rebound hyperexcitability when the drug is withdrawn. The withdrawal syndrome includes craving, anxiety, dysphoria, yawning, sweating, piloerection (waves of gooseflesh), lacrimation, rhinorrhoea, insomnia, nausea or vomiting, diarrhoea, cramps, muscle aches, and fever. With short-acting drugs such as morphine or heroin, withdrawal symptoms may appear within 8-12 hours of the last dose of the drug, reach a peak at 48-72 hours, and clear after 7-10 days. With longer-acting drugs such as methadone, onset of withdrawal symptoms may not occur until 1-3 days after the last dose; symptoms peak between the third and eight day and may persist for several weeks, but are generally milder than those that follow morphine or heroin withdrawal after equivalent doses. (WHO Lexicon of alcohol and drug terms)

• Opioid, endogenous: Any one of the naturally occurring brain neuropeptides, which include at least two major groups, the enkephalins and the endorphins. Both can interact with opiate-binding sites (receptors) and may thus modulate the perception of pain; endorphins, in addition, appear to modulate mood and responses to stressful stimuli.