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.
The close call, however, prompted Congress to require more rigorous safety and efficacy data from drug manufacturers and empower the FDA to monitor the industry more closely. David Armstrong, ProPublica, 8 May 2025 To demand that every new formulation be tested against an inert placebo would mean turning back the clock to reproduce trials that may have taken place decades ago, but resulted in the licensing of the original vaccine after safety and efficacy were established. Michael Hiltzik, Los Angeles Times, 8 May 2025 Sometimes, the drugs fail to demonstrate safety or efficacy, while other times, clinical trials run out of funding or struggle to recruit enough patients. Alexis Kayser, MSNBC Newsweek, 7 May 2025 This is a man who has adopted as a career trajectory embracing some of the most noxious lies imaginable, those that cast doubt on the efficacy and safety of perhaps our greatest-ever medical achievement, vaccinations. New York Daily News Editorial Board, New York Daily News, 6 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 12 May. 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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