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 allows for dose customization but isn’t subject to the same reviews for safety, efficacy or consistency, and may carry added risks related to quality control, potency variation or contamination.
From Los Angeles Times • Jun. 23, 2026
And while experts don’t think A.I. will crack this one any time soon, they do expect that the efficacy of these A.I. carpet-bombing campaigns will increase as the technology continues to improve.
From Slate • Jun. 22, 2026
"Despite these promising results, we're continuing to move forward with new studies to more thoroughly evaluate the efficacy and safety of the system," she adds.
From Science Daily • Jun. 19, 2026
Put aside arguments on the reasoning and efficacy of the medical and scientific regime.
From The Wall Street Journal • Jun. 18, 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.
