Mode of Action

Opioid analgesics mimic endogenous opioids by activating opioid receptors in the central and peripheral nervous systems to produce analgesia, respiratory depression, sedation and constipation. They reduce transmission of the pain impulse by acting pre and post-synaptically in the spinal cord, and by modulating the descending inhibitory pathways from the brain. Cough suppression occurs in the medullary centre of the brain.

The affinity of individual opioid analgesics for receptors varies and opioids may act as pure agonists or partial agonists; see Opioid Comparative Information Table

Partial agonists demonstrate a ceiling response above which an increase in dose does not produce an additional increase in effect.