Opioids

Agonists at the μ, κ and δ opioid receptors, together with the antagonists that displace them. μ agonism carries the analgesia and the euphoria, and the same receptor slows breathing — the effects are not separable, which is why every family below shares one mechanism of death. The families split by what a compound is built from: the poppy morphinans, the thebaine-derived oripavines, and the fully synthetic scaffolds — fentanyls, nitazenes, benzamides, open-chain diphenylpropylamines — that share the receptor and almost nothing else.

Potency per unit mass, onset, duration and receptor selectivity vary enormously across those scaffolds, so nothing learned about one carries to another on the strength of the word opioid, and tolerance falls away during any break in use. Effect decides the path here, not the receptor: DXM is a morphinan and is filed under Dissociatives, and salvinorin A is a selective κ agonist filed under Atypical, because neither is experienced as an opioid.