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
While the popularity of these app-blocking devices increases (Novelli said the company has sold more than 60,000), questions remain about their efficacy when the people who buy them are mainly discovering the devices through the internet—and talking about them on social media.
—Marco Quiroz-Gutierrez, Fortune, 13 Feb. 2026
But there is little to no medical evidence as to its efficacy in treating COVID-19.
—Bestreviews, Chicago Tribune, 13 Feb. 2026
The efficacy of the joint survey and its response rates, the league argued, were compromised by multiple surveys covering similar topics.
—Michael McCann, Sportico.com, 13 Feb. 2026
Further testing in lab experiments and clinical trials will be needed to confirm the efficacy of such a drug.
—Khloe Quill, FOXNews.com, 12 Feb. 2026
See All Example Sentences for efficacy
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Marco Quiroz-Gutierrez, Fortune, 13 Feb. 2026
But there is little to no medical evidence as to its efficacy in treating COVID-19.
—Bestreviews, Chicago Tribune, 13 Feb. 2026
The efficacy of the joint survey and its response rates, the league argued, were compromised by multiple surveys covering similar topics.
—Michael McCann, Sportico.com, 13 Feb. 2026
Further testing in lab experiments and clinical trials will be needed to confirm the efficacy of such a drug.
—Khloe Quill, FOXNews.com, 12 Feb. 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 15 Feb. 2026.
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