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
And retatrutide's higher efficacy is likely best targeted for the population that needs to lose even more weight fast to alleviate other issues, like pain caused by knee osteoarthritis.
—Jeff Marks, CNBC, 30 Jan. 2026
That, along with her incredible skin, is truly a testament to their efficacy.
—Margaux Anbouba, Vogue, 29 Jan. 2026
How exactly districts plan to monitor the efficacy of AI in the classroom largely remains to be seen.
—Grace Tucker, Cincinnati Enquirer, 29 Jan. 2026
Phase 1 human clinical trials showed it was well tolerated, and Bakshi says early results in Phase 2, which is focused on both safety and efficacy, are promising.
—Ana Castelain, Bloomberg, 28 Jan. 2026
See All Example Sentences for efficacy
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Jeff Marks, CNBC, 30 Jan. 2026
That, along with her incredible skin, is truly a testament to their efficacy.
—Margaux Anbouba, Vogue, 29 Jan. 2026
How exactly districts plan to monitor the efficacy of AI in the classroom largely remains to be seen.
—Grace Tucker, Cincinnati Enquirer, 29 Jan. 2026
Phase 1 human clinical trials showed it was well tolerated, and Bakshi says early results in Phase 2, which is focused on both safety and efficacy, are promising.
—Ana Castelain, Bloomberg, 28 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 2 Feb. 2026.
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