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
Romvari’s images are distinguished by their dual sense of logical efficacy and aesthetic loft.
—Richard Brody, New Yorker, 12 Apr. 2026
In one study, the perception by users that social media is addictive, due mainly to believing news media narratives, actually reduces self-efficacy and makes overuse more likely.
—William Proctor, The Orlando Sentinel, 11 Apr. 2026
For instance, industry has distinct knowledge about AI’s emergent capabilities and risks, the efficacy of evaluations, and innovations in model training and alignment.
—Morgan Chalfant, semafor.com, 10 Apr. 2026
These claims contradict decades of scientific evidence that demonstrates the safety and efficacy of combination vaccines.
—Kar-Hai Chu, The Conversation, 10 Apr. 2026
See All Example Sentences for efficacy
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Richard Brody, New Yorker, 12 Apr. 2026
In one study, the perception by users that social media is addictive, due mainly to believing news media narratives, actually reduces self-efficacy and makes overuse more likely.
—William Proctor, The Orlando Sentinel, 11 Apr. 2026
For instance, industry has distinct knowledge about AI’s emergent capabilities and risks, the efficacy of evaluations, and innovations in model training and alignment.
—Morgan Chalfant, semafor.com, 10 Apr. 2026
These claims contradict decades of scientific evidence that demonstrates the safety and efficacy of combination vaccines.
—Kar-Hai Chu, The Conversation, 10 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 20 Apr. 2026.
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