efficacy

noun
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

Levonorgestrel decreases in efficacy over time and doesn’t work as effectively after a sharp uptick of luteinizing hormone (LH) tells one of your ovaries to release an egg for fertilization. Natasha Lavender, SELF, "7 Myths About Emergency Contraception You Should Stop Believing," 8 Apr. 2019 There are no warnings about efficacy and weight on the package of the emergency contraception Plan B even though evidence suggests that the morning-after pill might be less effective for heavier women. Macaela Mackenzie, Glamour, "What Shrill Gets Right—and Wrong—About the Morning-After Pill Weight Limit," 15 Mar. 2019 But generally speaking, the medical community’s concerns about the vaginal lightening trend have less to do with efficacy and more to do with safety. E.j. Dickson, Vox, "The dangerous rise of vaginal lightening," 6 Dec. 2018 The efficacy of products like scalp exfoliators and ph-balancing shampoos? Evelyn Crowley, Vogue, "The 6 Biggest Skincare Breakthroughs of 2018," 19 Dec. 2018 To maximize the efficacy of both of these active ingredients in your own skin-care routine, Zeichner recommends a salicylic acid cleanser, followed by an oil-free moisturizer, then topped off with a benzoyl peroxide spot treatment. Allure, "How to Treat Cystic Acne, According to Dermatologists," 14 Mar. 2019 This is because the efficacy of using your IUD past its deadline appears to depend largely on factors like your age and, relatedly, your fertility. Korin Miller, SELF, "When Is It OK to Get Your IUD Removed?," 12 Mar. 2019 Smoke detectors lose their efficacy after 10 years and require replacing just like carbon monoxide alarms. Caroline Picard, Good Housekeeping, "Why You Need to Check Your Smoke Detectors During Daylight Saving," 2 Nov. 2018 Another is that our wariness could limit their efficacy. Diana Gitig, Ars Technica, "Glyphosate is safe, but some scientists still question how we regulate it," 5 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

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

8 May 2019

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Time Traveler for efficacy

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

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More Definitions for efficacy

efficacy

noun

English Language Learners Definition of efficacy

formal : the power to produce a desired result or effect

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