GLOSSARY: Efficacy

Efficacy, or intrinsic activity (IA), is defined as the relative ability of a drug-receptor complex to produce a maximum functional response. This use of the word "efficacy" was introduced by Stephenson (1956) to describe the way in which agonists vary in the response they produce, even when they occupy the same number of receptors. High efficacy agonists can produce the maximal response of the receptor system while occupying a relatively low proportion of the receptors in that system.