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
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.
Leerink noted that the efficacy of the study exceeded the firm’s expectations, meaningfully de-risking the drug combination strategy.
From Barron's • 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
Early data from China showed “robust efficacy signals,” Dai noted.
From Barron's • May 27, 2026
Brain-imaging studies offer a possible explanation for EMDR’s efficacy.
From The Wall Street Journal • May 26, 2026
The meeting’s topic was the efficacy of radiation for medical treatments of all kinds, but the impression made on Ernest and John was very different.
From "Big Science" by Michael Hiltzik
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.
