efficacy
noun
ef·fi·ca·cy
ˈe-fi-kə-sē 
plural efficacies
: the power to produce a result or effect
increased teacher efficacy in classroom management
The efficacy of this treatment has not yet been proved.
In the upcoming … clinical trial, researchers will further investigate the efficacy and safety of the vaccine.—
Julia Landwehr
Julia LandwehrExamples 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
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
Additional trial data showed that a higher dose of the pill could deliver greater efficacy – more than 15% weight loss – surpassing the results seen with the highest dose of Eli Lilly’s orforglipron.
—Annika Kim Constantino, CNBC, 10 Jan. 2026
Of course, vaccine evaluation and monitoring should be constant, particularly for those administered to children, to ensure safety and efficacy.
—Jerome Adams, Fortune, 8 Jan. 2026
Yes, like most supplements, the ingredients will eventually lose their efficacy over time.
—Bestreviews, Chicago Tribune, 7 Jan. 2026
Individually, too, there are some lingering concerns about the efficacy of Nix’s front.
—Luca Evans, Denver Post, 7 Jan. 2026
See All Example Sentences for efficacy
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Annika Kim Constantino, CNBC, 10 Jan. 2026
Of course, vaccine evaluation and monitoring should be constant, particularly for those administered to children, to ensure safety and efficacy.
—Jerome Adams, Fortune, 8 Jan. 2026
Yes, like most supplements, the ingredients will eventually lose their efficacy over time.
—Bestreviews, Chicago Tribune, 7 Jan. 2026
Individually, too, there are some lingering concerns about the efficacy of Nix’s front.
—Luca Evans, Denver Post, 7 Jan. 2026
See All Example Sentences for efficacy
Cite this Entry
“Efficacy.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/efficacy. Accessed 11 Jan. 2026.
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