ef·​fi·​ca·​cy | \ˈe-fi-kə-sē \
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 experts say there’s no proof of their safety or efficacy and, like all supplements, they aren’t regulated by the FDA. Optimizing sleep. Maridel Reyes, Glamour, "'Biohacking' Your Anxiety Is a Buzzy Practice—but What Does It Actually Mean?," 16 Oct. 2018 According to a press release from the FDA, the ring was studied within several clinical trials where it was tested for safety and efficacy. De Elizabeth, Teen Vogue, "FDA Approves Contraceptive Vaginal Ring That Can Be Used for a Year," 11 Aug. 2018 But the agency retains oversight of the treatments, including the physicians administering them, and can use the results of these treatments to judge the efficacy and safety of the experimental drugs. Michael Hiltzik, latimes.com, "The Koch network uses a flagrant falsehood to defend a dangerous and irresponsible law," 9 Feb. 2018 The jury is still out, for example, on the efficacy of unconscious bias training — workshops that help managers and employees be more aware of their prejudices. Tracey Lien, latimes.com, "As diversity progress in Silicon Valley stalls, advocates call for a new approach," 4 June 2018 Safety- and efficacy-wise, the monthly oral medications are way superior. Patty Khuly, miamiherald, "Simple tips for whether (or not) your dog needs frequent baths | Miami Herald," 25 Jan. 2018 Ultra-Ever Dry's efficacy depends greatly on how it's applied. Lee Hutchinson, Ars Technica, "The Internet-demanded, partially scientific testing of Ultra-Ever Dry (in HD!)," 4 July 2018 The tension was expressed in popular media as a defense of American individualism against the power and efficacy of Japanese collectivism and corporate culture. Lindsay Grace, Smithsonian, "The Original ‘Space Invaders’ is an Icon of 1970s America’s Deepest Fears," 19 June 2018 Alternative, engineered fixes have been dogged by potentially stratospheric costs, uncertain efficacy or both. The Economist, "Extracting carbon dioxide from the air is possible. But at what cost?," 7 June 2018

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

see efficacious

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Statistics for efficacy

Last Updated

4 Dec 2018

Look-up Popularity

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

: 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 nest or breeding place

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