Opioids, which can be naturally occurring, semisynthetic or synthetic compounds, produce analgesic effects by binding to opioid receptors in the central nervous system. Several different opioid receptors have been identified and, based on their interactions with these receptors, opioids fall into three main categories:
Pure agonists – drugs that bind to and stimulate opioid receptors, and are capable of producing a maximal response
Partial agonists – drugs that stimulate opioid receptors but have a ceiling effect, i.e. produce a submaximal response compared with an agonist
Mixed agonist-antagonists – drugs that are agonists for one opioid receptor but antagonise other opioid receptors.
Opioids are also classified as being ‘strong’ or ‘weak’, depending on the strength of their clinical effect, which has historically been measured against the effect of morphine.
Some opioids and their classifications are listed below: