Efficacy

Efficacy is the capacity for beneficial change (or therapeutic effect) of a given intervention, most commonly used in the practice of medicine and pharmacology.

Medicine

In medicine, efficacy is the capacity for beneficial change (or therapeutic effect) of a given intervention (for example a drug, medical device, surgical procedure, or a public health intervention). If efficacy is established, an intervention is likely to be at least as good as other available interventions, with which it will have been compared. Comparisons of this type are typically made in 'explanatory' randomized controlled trials, whereas 'pragmatic' trials are used to establish the effectiveness of an intervention.

When talking in terms of efficacy vs. effectiveness, effectiveness relates to how well a treatment works in the practice of medicine, as opposed to efficacy, which measures how well treatment works in clinical trials or laboratory studies.

Pharmacology

In pharmacology, efficacy (E_max) is the maximum response achievable from a drug.Intrinsic activity is a relative term that describes a drug's efficacy relative to a drug with the highest observed efficacy. Effectiveness refers to the ability of a drug to produce a beneficial effect. A distinction is made between 'method' effectiveness which describes the effect achievable if the drug was taken as prescribed and 'use' effectiveness which is the effect obtained under typical use circumstances when adherence is not 100%. The widely used intention to treat method of analysing clinical trials provides estimates of 'use' effectiveness which are typically biased compared with 'method' effectiveness. Efficacy is the magnitude of response with respect to the K_d.

Intrinsic activity

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.

Agonists of lower efficacy are not as efficient at producing a response from the drug-bound receptor, by stabilizing the active form of the drug-bound receptor. Therefore, they may not be able to produce the same maximal response, even when they occupy the entire receptor population, as the efficiency of transformation of the inactive form of the drug-receptor complex to the active drug-receptor complex may not be high enough to evoke a maximal response. Since the observed response may be less than maximal in systems with no spare receptor reserve, some low efficacy agonists are referred to as partial agonists.