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.
Hope Florida Foundation got a new board and filed tax returns, but criticisms of its efficacy still dog its leaders. — Christopher Spata, Miami Herald, 17 Oct. 2026 In oncology, where treatment efficacy hinges on precise, patient‑specific data, even a small AI hallucination, such as a mistimed test, can misdirect care. — Brad Kreick, Forbes.com, 16 Oct. 2026 That is the window in which the field’s claims about fewer side effects, meaningful efficacy at low circulating doses and consistent oral absorption either hold up or don’t. — Samantha Agate, Kansas City Star, 16 Oct. 2026 Data from two phase three trials evaluating the efficacy of litifilimab will be published in the fourth quarter of 2026. — Liz Napolitano, CNBC, 12 Oct. 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 20 Oct. 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

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