In pharmacology, efficacy (Emax) is the maximum response achievable from an applied or dosed agent, for instance, a small molecule drug.
The target of a drug is commonly referred to as a receptor, but can in general be any chemically sensitive site on any molecule found in the body.
The nature of such binding can be quantified by characterising how tightly these molecules, the drug and its receptor, interact: this is known as the affinity.
[9] Specifically, efficacy refers to "whether a drug demonstrates a health benefit over a placebo or other intervention when tested in an ideal situation, such as a tightly controlled clinical trial.
[citation needed] Effectiveness refers to "how the drug works in a real-world situation",[11] and is "often lower than efficacy because of interactions with other medications or health conditions of the patient, sufficient dose or duration of use not prescribed by the physician or followed by the patient, or use for an off-label condition that had not been tested.
"[22] The Formula of Concord teaches that when humans reject the calling of the Holy Spirit, it is not a result of the Word being less efficacious.