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

The lack of clarity regarding weight and the efficacy of emergency contraceptives is daunting, but Dr. Edelman notes that researchers are recruiting subjects for studies to answer this very question. Brittney Mcnamara, Allure, "Shrill Is Right About the Impact of Your Weight on the Morning-After Pill's Efficacy," 25 Mar. 2019 Yet rigorous empirical evidence demonstrating the efficacy of these programs is scarce. Jason Lyall, Washington Post, "To win 'hearts and minds' in Afghanistan, some aid programs worked better than others," 19 Feb. 2018 The reasoning goes that application order dictates how various products are absorbed, and therefore, their efficacy. A.a. Newton, SELF, "How Much Does the Order of Your Skin-Care Routine Actually Matter?," 11 June 2019 That aspect seems to be essential to its efficacy: several studies have shown that the more profoundly mystical the experience, the greater the therapeutic effect. The Economist, "Magic mushrooms, illegal in most places, may have therapeutic uses," 7 June 2019 If at any point the vaccine’s temperature rises to room temperature, its efficacy drops dramatically. Kelsey Piper, Vox, "Vaccines that don’t need refrigeration could save a ton of lives," 5 June 2019 But critics note that the drug was approved for many of those conditions long before the FDA's more rigorous standards of today and that there are few randomized clinical studies showing its efficacy, especially in adult conditions. Wayne Drash, CNN, "Mallinckrodt paid 'kickbacks' to keep drug price high, and stuck taxpayers with bill, DOJ lawsuit says," 5 June 2019 Instead, the anger has been in response to something else: an emphasis on roughing the passer, which has required weekly explanations in response to weekly head-scratching about its efficacy. Andrew Beaton, WSJ, "The NFL’s Grumpy Season," 6 Oct. 2018 Last year, Prime Minister Narendra Modi announced an $87 billion plan to link 60 rivers across India in an attempt to provide better irrigation sources and reduce flooding, though its efficacy remains to be seen. Kamakshi Ayyar, Time, "The Global Water Crisis: Why Are India’s Taps Running Dry?," 6 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

3 Jul 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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