efficacy

noun

ef·​fi·​ca·​cy ˈe-fi-kə-sē How to pronounce efficacy (audio)
plural efficacies
: the power to produce an effect

Examples of efficacy in a Sentence

In the planner's view, Rumsfeld had two goals: to demonstrate the efficacy of precision bombing and to "do the war on the cheap." Seymour M. Hersh, New Yorker, 7 Apr. 2003
Vaccines exist, but their efficacy against aerosolized plague is unknown. Sharon Begley et al., Newsweek, 8 Oct. 2001
efficacy does not have to be demonstrated before homeopathic products are marketed. Alison Abbott et al., Nature, 26 Sept. 1996
questioned the efficacy of the alarms in actually preventing auto theft
Recent Examples on the Web
Examples are automatically compiled from online sources to show current usage. Read More Opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback.
Harvard sociologist and Assistant Professor of Public Policy Liz McKenna said that in the past, movements of this scale have succeeded in influencing social change, but that their efficacy has dropped significantly since the turn of the century. NPR, 18 Oct. 2025 The implication of this inequity in clinical trial representation is that treatments that are provided to women in the real world may lack the same level of evidence as for men, posing issues of treatment safety and efficacy. Harsh Gupta, Time, 16 Oct. 2025 Esfandyar-pour said the platform could deepen understanding of cancer mechanisms and improve predictive accuracy in drug efficacy assessments. Aamir Khollam, Interesting Engineering, 15 Oct. 2025 Each disease requires trials to show efficacy of treatment to the FDA. Noah Lyons, San Diego Union-Tribune, 15 Oct. 2025 See All Example Sentences for efficacy

Word History

Etymology

borrowed from Latin efficācia, from efficāc-, efficāx "capable of fulfilling a function, efficacious" + -ia -y entry 2

First Known Use

1527, in the meaning defined above

Time Traveler
The first known use of efficacy was in 1527

Browse Nearby Words

Cite this Entry

“Efficacy.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/efficacy. Accessed 19 Oct. 2025.

Kids Definition

efficacy

noun
ef·​fi·​ca·​cy ˈef-i-kə-sē How to pronounce efficacy (audio)
plural efficacies
: the power to produce a desired result

More from Merriam-Webster on efficacy

Last Updated: - Updated example sentences
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