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.
Moreover, researchers have conducted thorough evaluations of the vaccines in pregnant women, confirming their safety and efficacy in this demographic. Dave Wessner, Forbes.com, 29 May 2025 Realistically, this Phase I trial – which tests for safety and efficacy, as well as dose parameters – should be completed by some time next year, and if successful will move to the next stage, involving a larger population. Bronwyn Thompson, New Atlas, 26 May 2025 The sunscreen also contains phytic acid, which gently exfoliates the skin, offers a brightening touch, and enhances the efficacy of the formula’s antioxidants. Jenny Berg, Vogue, 23 May 2025 Sunscreens are one of the only skin care products that are regulated by the FDA for efficacy, so they are required to have an expiration date and to last for at least three years unopened. Leslie Baumann, Miami Herald, 22 May 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 6 Jun. 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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