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.
Many have significant reservations about the safety and efficacy of the COVID vaccine, despite assurances from most experts that the benefits outweigh the risks. Paul Sisson, San Diego Union-Tribune, 20 Sep. 2025 Many of Kennedy’s picks have expressed skepticism about the safety and efficacy of vaccines. Erika Edwards, NBC news, 19 Sep. 2025 The opportunity lies in meeting demand with products that combine proven efficacy with the value of a beauty product rather than being treated as a simple commodity. Fairchild Studio, Footwear News, 19 Sep. 2025 Forgetting even just one pill can negatively impact efficacy. Pearl Pugh, Forbes.com, 19 Sep. 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 20 Sep. 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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