ef·​fi·​ca·​cy | \ ˈe-fi-kə-sē How to pronounce efficacy (audio) \
plural efficacies

Definition of efficacy

: the power to produce an effect

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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
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Recent Examples on the Web But all of that efficacy hasn't translated into the kinds of sales investors once hoped for. Sy Mukherjee, Fortune, "Novartis’ $9.7 Billion Deal Sets off Heart Drug Wars," 25 Nov. 2019 Brügger toys with the viewer like a postmodern Hitchcock, musing on his own lack of efficacy. Daphne Merkin, The New Republic, "Cold Case Hammarskjöld’s Artful Paranoia," 3 Sep. 2019 Beyond fears of toxicity, there is also the question of efficacy. Sara Harrison, WIRED, "The Wild, Unregulated World of Sports Supplements," 22 July 2019 Labeling would have to clearly indicate that the products do not have any scientific evidence of efficacy and that homeopathic notions are not accepted by the modern medical community. Beth Mole, Ars Technica, "Massive recall of homeopathic kids’ products spotlights dubious health claims," 24 Aug. 2018 In addition to efficacy, lawmakers need to examine the law’s impact on civil rights. Krista Kafer, The Denver Post, "Kafer: Let’s see how new gun law works before pushing for more," 28 Nov. 2019 The bride and groom are often able to take the reins, so coercion is lower and efficacy, higher. Ananya Bhattacharya, Quartz India, "Dil Mil, a dating app for the Indian diaspora, is redefining “arranged” marriages," 27 Nov. 2019 More studies are slated to continue to test the vaccine’s efficacy, including one early next year that will focus on Finnish travelers visiting Africa, according to Medical Xpress. Benjamin Vanhoose, PEOPLE.com, "Diarrhea Vaccine Found to Be Safe for Children, Scientists Say," 22 Nov. 2019 Expectations vary on the efficacy of such measures. BostonGlobe.com, "Not long after Senator Eric Lesser proposed his relocation incentive bill, the e-mails started rolling in.," 24 Sep. 2019

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'efficacy.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

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First Known Use of efficacy

13th century, in the meaning defined above

History and Etymology for efficacy

borrowed from Latin efficācia, from efficāc-, efficāx "capable of fulfilling a function, efficacious" + -ia -y entry 2

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Time Traveler for efficacy

Time Traveler

The first known use of efficacy was in the 13th century

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Statistics for efficacy

Last Updated

16 Jan 2020

Cite this Entry

“Efficacy.” The Merriam-Webster.com Dictionary, Merriam-Webster Inc., https://www.merriam-webster.com/dictionary/efficacy. Accessed 28 January 2020.

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More Definitions for efficacy


How to pronounce efficacy (audio)

English Language Learners Definition of efficacy

formal : the power to produce a desired result or effect

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Comments on efficacy

What made you want to look up efficacy? Please tell us where you read or heard it (including the quote, if possible).


showing steady, earnest care and effort

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