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.
Lilly's shot is the gold standard in terms of efficacy, Skovronsky said. Angelica Peebles, CNBC, 12 Sep. 2025 Larger trials are needed to confirm the efficacy of azelastine against COVID-19 and to explore the medication’s potential benefits against other respiratory illnesses across more diverse populations and settings, Meiser says. Tom Gavin, EverydayHealth.com, 11 Sep. 2025 With four generations in the workplace, understanding the efficacy and differences of communication styles is a must for any leader and a critical component of their leadership style. Kathryn Lancioni, Forbes.com, 11 Sep. 2025 Those with chronic pain reported significantly higher levels of perfectionism and lower levels of self-compassion and self-efficacy than the pain-free group. New Atlas, 10 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 15 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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