The degree to which a method or medicine brings about a specific result is its efficacy. You might not like to eat it, but you can't question the efficacy of broccoli as a health benefit.

Efficacy is a more formal way to say effectiveness, both of which stem from the Latin verb efficere "to work out, accomplish." The effectiveness, or efficacy, of something is how well it works or brings the results you hoped for. A scientist does research to determine the efficacy of a vaccine or medicine under development. If it is efficacious, it will cure or prevent a disease.

Definitions of efficacy

n capacity or power to produce a desired effect

“concern about the safety and efficacy of the vaccine”
inefficaciousness, inefficacy
a lack of efficacy
Type of:
effectiveness, effectivity, effectuality, effectualness
power to be effective; the quality of being able to bring about an effect

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