efficacy /ef″əkəsē/ [L, effectus, performance] , 1. (of a drug or treatment) the ability of a drug or treatment to produce a specific result, regardless of dosage. 2. (in psychology) the personal ability to act upon and react to the world to achieve desired results. 3. a person’s beliefs about whether he or she can use his or her capacities to influence the course of events or circumstances in the external world; efficacy refers to how effective one feels about his or her abilities. See also self-efficacy.
