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.
This is a big problem in terms of the Presidents efficacy as well as well as his administration. Hollie Silverman, MSNBC Newsweek, 9 June 2025 Safety and efficacy vary highly between products; look for those that are independently tested and verified. Sarah Anzlovar, Verywell Health, 9 June 2025 Here’s what to know about the new report, whether you should be concerned about the safety and efficacy of your favorite sunscreen and how to best protect your skin. Simon Spichak, Health, 4 June 2025 In a letter signed by a bipartisan group of 17 Assembly members, Josh Hoover, R-Folsom, and Robert Garcia, D-Rancho Cucamonga, urged the governor to push back the timeline for returning employees until the California State Auditor publishes a report analyzing the cost and efficacy of telework. William Melhado, Sacbee.com, 4 June 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 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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