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.
The study, which has not yet been peer-reviewed, involved 40 patients and tested both safety and efficacy of the treatment. Amy Feldman, Forbes.com, 9 Apr. 2025 The authors recommend that the treatment be tried with many more patients to test both the approach’s safety and efficacy before it is approved for widespread use. Paul Smaglik, Discover Magazine, 8 Apr. 2025 This measles death could portend more to come without clear government communication about the safety and efficacy of vaccines. John W. Dean, MSNBC Newsweek, 7 Apr. 2025 Kennedy, who has a controversial history of questioning the safety and efficacy of vaccines, including falsely linking them to autism, has said people are being successfully treated with Vitamin A and cod liver oil for the highly contagious respiratory illness. David Oliver, USA Today, 7 Apr. 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 21 Apr. 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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