Efficacy is the ability to produce a desired amount of a desired effect.
In a medical context it indicates that the therapeutic effect of a given intervention (e.g. intake of a medicine, an operation, or a public health measure) is acceptable. 'Acceptable' in that context refers to a consensus that it is at least as good as other available interventions to which it will have ideally been compared to in a clinical trial. For example, an efficacious vaccine has the ability to prevent or cure a specific illness in an acceptable proportion of exposed individuals. In strict epidemiological language, 'efficacy' refers to the impact of an intervention in a clinical trial, differing from 'effectiveness' which refers to the impact in real world situations.
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.
- agonist: affinity and maximum efficacy
- antagonist : affinity without efficacy
- partial agonist: affinity and partial efficacy
In lighting design, "efficacy" refers to the amount of light (luminous flux) produced by a lamp (a light bulb or other light source), usually measured in lumens, as a ratio of the amount of energy consumed to produce it, usually measured in watts. This is not to be confused with efficiency which is always a dimensionless ratio of output divided by input which for lighting relates to the watts of visible energy as a ratio of the energy consumed in watts. The visible energy can be approximated by the area under the Plancks curve between 300 nm and 700 nm for a blackbody at the temperature of the filament as a ratio of the total energy under the blackbody curve. Efficiency values for light from a heat source are typically less than two percent.
The efficacy of a differential amplifier is measured by the degree of its rejection of common-mode signals inpreference to differential signals. Refer to common-mode rejection ratio (CMRR).
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- Placebo (origins of technical term)
- Vaccine efficacy
- Figure of merit
- Stephensn RP 1965 A modificaion of receptor theory. Br J Pharmacol 11:379-393