efficacy
Americannoun
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the capacity for producing a desired result or effect.
Short, frequent periods of practice were shown to have greater efficacy than longer and less frequent ones.
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a measure of the success of a vaccine or other pharmaceutical when used in the controlled environment of a clinical trial, as opposed to in the real world.
noun
Other Word Forms
Derived Forms
Inflected Forms
Nouns
Etymology
Origin of efficacy
First recorded in 1520–30; from Latin efficācia, from efficāc- (stem of efficāx ) “effective, effectual” + -ia, noun suffix; see efficacious ( def. ), -y 3 ( def. )
Explanation
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.
Vocabulary lists containing efficacy
ASVAB Word Knowledge
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Power Suffix: -acy
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This Week in Words: December 30, 2017 - January 5, 2018
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Example Sentences
Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
"This lets us speak about safety and efficacy with real confidence," he said.
From Science Daily • Jun. 17, 2026
The company reported positive results for two studies of its elecoglipron experimental drug, which showed competitive efficacy with a standard safety profile, the analysts say.
From The Wall Street Journal • Jun. 9, 2026
The analysts at Citi also highlighted what they said were real-world signals of waning efficacy in Wegovy pill as roughly a quarter to a third of patients are no longer losing weight.
From MarketWatch • Jun. 8, 2026
They add: "Assessing long-term efficacy is critical since those who work night shifts for many years would need to consistently consume melatonin supplements over that time frame to maximize the potential cancer prevention benefits."
From Science Daily • May 30, 2026
This discipline is of such efficacy that not only does it maintain those who were born princes, but it enables men of private station on many occasions to rise to that position.
From "The Prince" by Niccolò Machiavelli
Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
