Intrinsic activity (
IA) or
efficacy refers to the relative ability of a
drug-
receptor complex to produce a maximum functional response. This must be distinguished from the
affinity, which is a measure of the ability of the drug to bind to its molecular target, and the
EC50, which is a measure of the
potency of the drug and which is proportional to both efficacy and affinity. 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.