Agonists, antagonists and partial agonists

Agonists

An agonist binds and activates, producing the receptor's response. A full agonist produces the maximum response the system can give; a partial agonist produces less even at saturating occupancy, because activation efficiency is lower.

Antagonists

An antagonist binds without activating, and by occupying the site prevents an agonist from acting. Competitive antagonism can be overcome by more agonist; non-competitive antagonism cannot, because the antagonist acts elsewhere on the receptor.

Biased agonism

Receptors frequently couple to more than one downstream pathway. A biased agonist activates some more than others, so two agonists at the same receptor can produce different downstream profiles. This is an active area of research and one reason "agonist" alone under-describes a molecule.

Multi-target molecules

A peptide acting at several receptors has a profile at each — full at one, partial at another. Describing such a molecule requires stating what it does at each target, not simply listing them.