Intrinsic activity

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Intrinsic activity (IA) and efficacy refer 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)^[1] 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. There is a distinction between efficacy and intrinsic activity.^{[clarification needed]}