ef·​fi·​ca·​cy ˈe-fi-kə-sē How to pronounce efficacy (audio)
plural efficacies
: the power to produce an effect

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
Recent Examples on the Web Sanders said the bills were necessary to facilitate government efficacy, claiming activists have weaponized the state's Freedom of Information Act to slow her agenda. Neal Earley, Arkansas Online, 14 Sep. 2023 The study looked at antibody levels against the flu and didn’t directly assess efficacy in preventing disease symptoms. Robert Langreth, BostonGlobe.com, 13 Sep. 2023 The advisory committee’s decision isn’t binding, but the FDA’s own staff have also questioned phenylephrine’s efficacy. Cailley Lapara, Fortune, 12 Sep. 2023 Will my NorCal twitter troll have the courage and efficacy to become my dem whip?? Reese Gorman, Washington Examiner, 11 Sep. 2023 Gaga has hit the nail on the head—consumers want ingredients that are good for the skin without compromising efficacy. Kiana Murden, Vogue, 7 Sep. 2023 Each delicious gummy contains 50mg of our premium Delta 8 compound, which has been rigorously tested for purity and efficacy. Amber Smith, Discover Magazine, 3 Sep. 2023 Although, there's no evidence that alcohol affects the efficacy of the booster and vaccine. Claire Gillespie, Health, 31 Aug. 2023 This interaction could potentially increase or decrease the efficacy of certain drugs. Amber Smith, Discover Magazine, 30 Aug. 2023 See More

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'efficacy.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History


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

First Known Use

13th century, in the meaning defined above

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

Dictionary Entries Near efficacy

Cite this Entry

“Efficacy.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/efficacy. Accessed 22 Sep. 2023.

Kids Definition


ef·​fi·​ca·​cy ˈef-i-kə-sē How to pronounce efficacy (audio)
plural efficacies
: the power to produce a desired result

More from Merriam-Webster on efficacy

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