Partial agonist

A partial agonist is a molecule that activates a receptor but produces a smaller maximum response than a full agonist, even when it occupies every receptor.

Pharmacologists distinguish affinity, how well a ligand binds, from efficacy, how much it does once bound. A partial agonist may bind tightly yet have low efficacy, so its concentration-response curve plateaus below the full agonist's. A consequence is that in the presence of a full agonist, a partial agonist competes for the same sites and lowers the overall response, acting like a partial blocker; buprenorphine at opioid receptors is the standard example, valued because its ceiling limits respiratory depression.

THC is a partial agonist at CB1 and CB2 receptors, which is one reason its effects, though clearly intoxicating, plateau rather than escalating the way full synthetic agonists found in some illicit preparations do. CBD is not usefully described as a partial agonist at cannabinoid receptors; its binding there is weak and its actions are generally attributed to other targets, a distinction still being clarified in research.