The extent to which a specific intervention, procedure, regimen, or service produces a beneficial result under ideal conditions. Cf.:effectiveness. [L. efficacia, fr, ef-ficio, to perform, accomplish]
* * *
ef·fi·ca·cy (efґĭ-kə-se) [L. efficax effectual] 1. the ability of an intervention to produce the desired beneficial effect in expert hands and under ideal circumstances. Cf. effectiveness. 2. in pharmacology, the ability of a drug to produce the desired therapeutic effect; it is independent of potency, which expresses the amount of the drug necessary to achieve the desired effect.
Dose-effect curve for two drugs of different efficacy: The efficacy of drug A is greater than that of drug B.
Medical dictionary. 2011.