efficacy
Americannoun
plural
efficacies-
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.
-
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
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.
After reports that vitamin K deficiency bleeding was on the rise, the American Academy of Pediatrics updated its policy statement in 2022 to stress the shot’s safety and efficacy.
From Salon • May 7, 2026
The choice was rational: Long-term safety data was unavailable, efficacy thresholds hadn’t been cleared, and further investment in an unproven compound risked starving proven therapies of resources.
From The Wall Street Journal • May 5, 2026
“Our confidence in the safety and efficacy of Foundayo is based on our robust clinical trial program, which followed over 11,000 participants for up to two years,” the Lilly spokesperson told Barron’s.
From Barron's • May 4, 2026
The problem with this statement is that it ignores a long history demonstrating the drug’s safety and efficacy.
From Slate • May 4, 2026
Three were scientists checking the efficacy of sprays; 8 worked in greenhouses; 5 were farm workers.
From "Silent Spring" by Rachel Carson
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.
