In a healthcare context, efficacy indicates the capacity for beneficial change (or therapeutic effect) of a given intervention (e.g. a medicine, 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, to 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.
The concept of 'self-efficacy' is an important one in the self-management of chronic diseases because doctors and patients often do not follow best practice in using a treatment. For instance, a patient using combined oral contraceptive pills to prevent pregnancy may sometimes forget to take a pill at the prescribed time; thus, while the perfect-use failure rate for this form of contraception in the first year of use is just 0.3%, the typical-use failure rate is 8%.
In pharmacology, intrinsic activity or efficacy refers to the ability of a drug to induce a biological response in its molecular target. This must be distinguished from the affinity, which refers to the ability of the drug to bind to its molecular target. The term 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 their maximal response while occupying a relatively low proportion of receptors; agonists of lower efficacy cannot activate the receptors to the same degree and may not be able to produce the same maximal response even when they occupy the entire receptor population, thereby behaving as partial agonists
The term is often used to classify the activity of a drug upon binding to its receptor.
The efficacy of a differential amplifier is measured by the degree of its rejection of common-mode signals in preference to differential signals. Referred to as common-mode rejection ratio (CMRR); typically specified in decibels.