Main Difference – Efficacy vs Effectiveness
Although the two terms Efficacy and Effectiveness have the same meaning in popular usage, there is a distinctive difference between these two terms in the field of health and medicine. In general parlance, both efficacy and effectiveness refers to an ability to produce a desired or intended result. But in medical parlance, effectiveness refers to a result acquired in an average clinical or a real world environment whereas efficacy refers to a result acquired under ideal or controlled conditions. This is the main difference between efficacy and effectiveness in the field of medicine.
This article covers,
1. What Does Efficacy Mean – Definition, Meaning, and Usage
2. What Does Effectiveness Mean – Definition, Meaning, and Usage
3. Difference Between Efficacy and Effectiveness 
Efficacy – Definition, Meaning, and Usage
As mentioned above, efficacy can be used as a synonym for effectiveness in common parlance. The difference between efficacy and effectiveness exists only in medical parlance. Let’s look at some definitions of efficacy as given in medical dictionaries.
Farlex Partner Medical Dictionary:
“The extent to which a specific intervention, procedure, regimen, or service produces a beneficial result under ideal conditions.”
Mosby’s Dental Dictionary, (2nd edition):
“The ability to provide a clinically measurable effect, preferably beneficial.”
As seen from these definitions, efficacy refers to how well something works in an ideal or controlled setting such as a clinical trial. The patient population and other variable factors can be controlled in this setting. The end result or effect is also measurable. Efficacy measures how well a drug produces its desired result.
Effectiveness – Definition, Meaning, and Usage
Effectiveness refers to the ability to produce a decided, decisive, or desired effect. In medicine, there is an additional meaning to this word. Let’s look at the definition of efficacy in some medical dictionaries to understand this additional meaning.
Farlex Partner Medical Dictionary:
“A measure of the accuracy or success of a diagnostic or therapeutic technique when carried out in an average clinical environment.”
Mosby’s Dental Dictionary, (2nd edition):
“The degree to which action(s) achieves the intended health result under normal or usual circumstances.”
As seen from these two definitions, effectiveness describes how medication or treatments are used in a real-world or in average clinical conditions. The patient population and other variables cannot be controlled in this setting. The term effectiveness also refers to how easy a drug is to use and possible side effects of that drug.
Difference Between Efficacy and Effectiveness
Both efficacy and effectiveness refer to the ability to produce a decided, decisive, or desired effect. The following differences are only relevant for the field of medicine.
Definition
Efficacy refers to the degree to which action(s) produce a clinically measurable effect under ideal conditions.
Effectiveness refers to the degree to which action(s) achieves the intended health result under normal or usual circumstances.
Conditions
Efficacy describes how medications are used in controlled and ideal conditions.
Effectiveness describes how medications are used in average clinical conditions.
Measurable Effects
Efficacy refers to the ability to gain a clinically measurable effect.
Effectiveness does not refer to a clinically measurable effect.
Implications
Efficacy measures how well a drug produces its desired result.
Effectiveness refers to how easy a drug is to use and possible side effects of that drug.
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