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.
But prorating three years of player profits down to 12 months, as UEFA’s rule does, reduces the immediate efficacy of successful trading in the market, the very strategy that poorer clubs increasingly rely on to climb the ladder. Chris Weatherspoon, New York Times, 5 Sep. 2025 And no longevity-claiming therapy is considered fruitful without sound research that confirms efficacy after following a diverse cohort of humans for decades to see if the therapy is associated with extended life or staving off chronic disease. Alexa Mikhail, Flow Space, 5 Sep. 2025 Thanks to its ease, efficacy, and film-free finish, Pavitt loves micellar water for acne-prone skin types. Iman Balagam, Vogue, 3 Sep. 2025 In a makeup context, the question became how to preserve this full efficacy while also introducing color. Celia Shatzman, Forbes.com, 3 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 9 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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