A measure of the magnitude of drug response following interaction with the receptor. It can also reflect the number of drug–receptor complexes adopting the active state. Emax is a parameter that describes the maximum response elicited by the drug (maximal efficacy).
A full agonist is a drug that elicit maximal response (Emax = 100%), while partial agonists have lower Emax, even when 100% of the receptors are occupied by the drug. Antagonists have Emax equal to zero as the receptors are not activated. Efficacy is considered a more clinically useful parameter than drug potency, since a drug with higher efficacy is more therapeutically beneficial than is one that is more potent.
There are two main types of studies that can evaluate drug therapeutic response: observational studies and randomized controlled trials. In the latter, patients are randomly assigned to active or control group while observational studies (pragmatic trials or real-world trials) do not require randomization.
In this context, efficacy can be defined as the capacity of a given drug to exert its therapeutic effects under ideal conditions or in randomized controlled clinical trials. Therapeutic guidelines are ideally based on evidence from randomized controlled clinical trials or, occasionally on well-designed observational studies.
Efficacy should be distinguished from Effectiveness. Effectiveness is primarily estimated in observational studies. In real practice studies there are other variables that contribute to drug therapeutic effectiveness such as interactions with other medications and interactions with patient’s health conditions. Therefore, Effectiveness can be defined as the extent to which a drug achieves its intended therapeutic effect in the usual clinical setting