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.
Huynh described the phase 3 results as clinically meaningful and likely to support regulatory approval, but said the primary efficacy result landed at the low end of investor expectations. — Annika Kim Constantino, CNBC, 23 Oct. 2026 While that efficacy doesn’t match the performance of a rival drug from Roche, Novartis’ treatment has a cleaner safety profile that could give it a shot at being competitive in the market. — Andrew Joseph, STAT, 23 Oct. 2026 Right now, research about the safety and efficacy of estrogen skin cream just hasn’t caught up to the evidence on other kinds of hormonal therapy. — Ashley Abramson, Allure, 22 Oct. 2026 Anyone hoping for timely debate over the purpose or efficacy of the criminal justice system is bound to be disappointed. — Judy Berman, Time, 21 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 27 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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