efficacy

noun

ef·​fi·​ca·​cy ˈe-fi-kə-sē How to pronounce efficacy (audio)
plural efficacies
Synonyms of efficacy
: the power to produce a result or effect
increased teacher efficacy in classroom management
The efficacy of this treatment has not yet been proved.
In the upcoming … clinical trial, researchers will further investigate the efficacy and safety of the vaccine.—Julia Landwehr

Synonyms of efficacy

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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.
Political and security fractures will further reduce the efficacy of air defense networks. — Dina Esfandiary, Foreign Affairs, 27 Aug. 2026 Without needing to establish safety or efficacy, researchers can skip ahead to trials testing their broader potential. — Mike McRae 26, New Atlas, 26 Aug. 2026 But over time, people’s loyalty to online brands may be supplanted by the efficacy of their interfaces for agents. — George C. Lee Ii, Fortune, 26 Aug. 2026 Marston’s company has applied for grants to evaluate the efficacy of its drones. — Cindy Von Quednow, CNN Money, 24 Aug. 2026 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

Rhymes for efficacy

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Browse Nearby Words

efficacity
efficacy
efficience
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Cite this Entry

“Efficacy.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/efficacy. Accessed 1 Sep. 2026.

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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

Britannica.com: Encyclopedia article about efficacy

Last Updated: - Updated example sentences
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