Naturally occuring alkaloids, from the opium poppy Papaver somniferum, that are very effective painkillers but tend to be addictive and are drugs of abuse. Modified opium derivatives are opiates, including morphine, but opioid is a more general term covering natural, semisynthetic, and synthetic compounds. Weak opioids include codeine and dihydrocodeine; strong opioids include morphine, diamorphine, fentanyl, and phenazocine. Opioid receptors fall into four distinct classes: δ, μ, and κ together with an orphan receptor, the ligand for which is orphanin FQ. All are G-protein coupled and the endogenous ligands are enkephalins and endorphins.
Subjects: Clinical Pharmacology and Therapeutics.