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
These are the only doses that have been studied rigorously for safety and efficacy.
—Jody Dushay, STAT, 29 May 2026
For years, even as researchers challenged the long-term safety and efficacy of these expensive procedures, the federal government did little to stop potential abuse.
—Annie Waldman, ProPublica, 29 May 2026
There’s value in the ethos, and the Hurricanes’ status — one win from the Stanley Cup Final, one win from scaling the hill that comes before the mountain — is proof of its efficacy.
—Sean Gentille, New York Times, 28 May 2026
Feigenbaum said that people using these drugs in spite of their limited efficacy and unknown risks are made up of two populations.
—T.m. Brown, CNN Money, 28 May 2026
See All Example Sentences for efficacy
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Jody Dushay, STAT, 29 May 2026
For years, even as researchers challenged the long-term safety and efficacy of these expensive procedures, the federal government did little to stop potential abuse.
—Annie Waldman, ProPublica, 29 May 2026
There’s value in the ethos, and the Hurricanes’ status — one win from the Stanley Cup Final, one win from scaling the hill that comes before the mountain — is proof of its efficacy.
—Sean Gentille, New York Times, 28 May 2026
Feigenbaum said that people using these drugs in spite of their limited efficacy and unknown risks are made up of two populations.
—T.m. Brown, CNN Money, 28 May 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 1 Jun. 2026.
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