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An opioid is a substance acting at opioid receptors, including compounds with different structures and receptor effects. Morphine is an agonist example, while naloxone is an antagonist. Receptor action, analgesia and dependence are related but distinct concepts.

An opioid receptor is a G protein-coupled receptor recognizing opioid ligands, including endogenous peptides and exogenous drugs. Major receptor classes differ in distribution and ligand responses. Their signalling commonly engages inhibitory G proteins, linking receptor activation to changes in cyclic-AMP production and ion-channel activity. ^[ref]

The same bound ligand need not produce identical effects in every assay. Receptor abundance, coupling efficiency and downstream amplification affect observed efficacy. Experiments comparing candidate opioid agonists showed why low intrinsic G-protein efficacy can help explain apparently favourable signalling profiles, rather than assuming that every difference establishes a uniquely selective pathway. ^[ref]