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
In a handful of preliminary trials, the drug has shown some efficacy in treating substance use disorders, and the new trial will investigate the drug’s use as a treatment for alcohol use disorder.
—Claire Cameron, Scientific American, 24 Apr. 2026
In Africa, where many nations gained independence and shifted to democratic governments since the 1990s, the regulatory and financial frameworks for community radio have had varying levels of efficacy.
—Encyclopedia Britannica, 23 Apr. 2026
One of the main reasons peptides have gained such momentum in skincare is their balance of efficacy and gentleness.
—Brianna Peters, Vogue, 22 Apr. 2026
That naturally raised questions, not about whether the laws of time and space had been upended but about the efficacy, and honesty, of the billing systems being used by the firms, the city or both.
—The Editorial Board, Chicago Tribune, 21 Apr. 2026
See All Example Sentences for efficacy
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Claire Cameron, Scientific American, 24 Apr. 2026
In Africa, where many nations gained independence and shifted to democratic governments since the 1990s, the regulatory and financial frameworks for community radio have had varying levels of efficacy.
—Encyclopedia Britannica, 23 Apr. 2026
One of the main reasons peptides have gained such momentum in skincare is their balance of efficacy and gentleness.
—Brianna Peters, Vogue, 22 Apr. 2026
That naturally raised questions, not about whether the laws of time and space had been upended but about the efficacy, and honesty, of the billing systems being used by the firms, the city or both.
—The Editorial Board, Chicago Tribune, 21 Apr. 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 26 Apr. 2026.
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