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 problem with this statement is that it ignores a long history demonstrating the drug’s safety and efficacy.
From Slate • May 4, 2026
In January, Moderna released five-year follow-up data evaluating the efficacy of intismeran autogene, an mRNA vaccine for patients with certain types of melanoma, in combination with Merck’s Keytruda.
From Barron's • May 1, 2026
"Given the OII's findings, this very much calls into question the efficacy and merit of the advice being given," he said.
From BBC • Apr. 29, 2026
In the 1960s and 1970s Francis L. Black, a virologist at Yale, conducted safety and efficacy tests among South American Indians of a new, improved measles vaccine.
From "1491" by Charles C. Mann
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.
