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 an even more powerful next generation of medicines is quickly approaching, churning out trial results – including some new ones Thursday – that leave current options in the dust in terms of efficacy.
—Meg Tirrell, CNN Money, 19 Mar. 2026
The best Korean sunscreens were arguably the gateway to K-beauty, though while Korean skincare has become commonplace, that doesn’t diminish SPF’s efficacy.
—Deanna Pai, Vogue, 19 Mar. 2026
These are studies typically run by clinical researchers in hospitals that can be started quickly and with less red tape than a typical drug trial, accelerating initial insights into therapeutic efficacy.
—Megan Molteni, STAT, 19 Mar. 2026
No crunch, greasy residue, or flakes here—just pure efficacy.
—Catharine Malzahn, Glamour, 18 Mar. 2026
See All Example Sentences for efficacy
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Meg Tirrell, CNN Money, 19 Mar. 2026
The best Korean sunscreens were arguably the gateway to K-beauty, though while Korean skincare has become commonplace, that doesn’t diminish SPF’s efficacy.
—Deanna Pai, Vogue, 19 Mar. 2026
These are studies typically run by clinical researchers in hospitals that can be started quickly and with less red tape than a typical drug trial, accelerating initial insights into therapeutic efficacy.
—Megan Molteni, STAT, 19 Mar. 2026
No crunch, greasy residue, or flakes here—just pure efficacy.
—Catharine Malzahn, Glamour, 18 Mar. 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 Mar. 2026.
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