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.
Kennedy is a longtime anti-vaccine advocate who is now using his position as the top US health official to spread misinformation and doubt about the safety and efficacy of vaccines. Beth Mole, ArsTechnica, 2 Apr. 2025 Kennedy, an environmental lawyer, has a history of sowing doubts about the safety and efficacy of vaccines. Manas Mishra, USA Today, 31 Mar. 2025 Fewer employees mean less manpower to evaluate the safety and efficacy of new drugs that could dramatically impact diseases like cancer, diabetes and heart failure—issues that affect millions of Americans. Omer Awan, Forbes.com, 30 Mar. 2025 Marks is credited with coining the name and concept for Operation Warp Speed, the effort during President Trump’s first term to rapidly manufacture vaccines while they were still being tested for safety and efficacy. Michael Casey and Matthew Perrone, Los Angeles Times, 29 Mar. 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 16 Apr. 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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