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

Today’s edition explores shifting attitudes on gun control, the human toll of climate change, a rare moment of unity in Brazil around pension reform, the efficacy of busing in education, and the thorny nature of emotional support animals. The Christian Science Monitor, "Climate gloom and innovation bloom," 8 Aug. 2019 But ViiV injectables are the only long-acting ARVs likely to come to market anytime soon; the new Merck compound must still pass through large-scale efficacy trials. Jon Cohen, Science | AAAS, "Simpler HIV treatment and prevention strategies take center stage," 24 July 2019 There are different efficacy rates for all of them. Phil Rockrohr, chicagotribune.com, "Shout Out: Gretchen Waitley, a Vernon Hills resident with multiple sclerosis and organizer of PuckMS fundraiser," 19 July 2019 Many individuals struggle to maintain individual, or what is known as intrapersonal, levels of things like motivation, self-control, confidence, and self-efficacy. Natt Garun, The Verge, "The future of fitness is together but alone," 18 July 2019 Research on the Rounds’ efficacy is limited, but encouraging. Corinne Purtill, Quartz at Work, "A simple strategy helps doctors fight burnout. Could it work for the rest of us?," 5 June 2019 The action is intended to demonstrate your concern and good manners, not your efficacy as a doorstop. Judith Martin, The Mercury News, "Miss Manners: These moochers think they’re entitled to our pricey wine," 24 July 2019 But the research is incomplete about its efficacy for treating things like animal anxiety and seizures, and veterinarians are not allowed to recommend CBD to patients (although a bill making its way through California’s Senate could change that). Laura Klivans, chicagotribune.com, "Legal weed’s a growing danger to dogs, so keep your canine out of your cannabis," 10 July 2019 New information from an earlier trial indicated that the drug, RG6042, showed the same signs of potential efficacy when given every two months, as when given every month. San Diego Union-Tribune, "Experimental Huntington’s disease drug can be given less frequently," 1 July 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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Statistics for efficacy

Last Updated

14 Aug 2019

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Time Traveler for efficacy

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

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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).


a period when something is suspended

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