Efficacy is the capacity to produce an effect. It has different specific meanings in different fields.
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
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
In pharmacology, efficacy, refers to the maximum response
achievable from a drug. It is often described by the parameter Emax.
Intrinsic activity is a relative term which describes the efficacy
relative to a drug which has 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'
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