Efficacy (Pharmacology)
In pharmacological terms, the maximum biological response that a drug can produce regardless of dose. A full agonist has greater efficacy than a partial agonist at the same receptor. This pharmacological definition differs from clinical efficacy, which refers to how well a drug works in treating a condition.
Technical Context
Pharmacological efficacy (intrinsic efficacy) reflects the quality of the receptor-drug interaction — specifically, the conformational change induced in the receptor and the magnitude of downstream signalling. A full agonist induces the optimal conformational change for maximum signalling (efficacy = 1). A partial agonist induces a suboptimal change (efficacy 0-1). An antagonist binds without inducing productive signalling (efficacy = 0). This receptor-level concept is distinct from clinical efficacy (how well a drug treats a disease in patients). A partial agonist with lower pharmacological efficacy may still be clinically effective if partial receptor activation is sufficient for the therapeutic goal.