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
From there the compound would need to clear safety and efficacy testing in clinical trials before it could ever be used with patients.
—Ryan Brennan, Sacbee.com, 18 June 2026
Operating ‘beyond the bounds of FDA approval’ Although there is a lack of FDA approval and little evidence of its efficacy, stem cell treatments for autism are being steadily provided across the country.
—Theara Coleman, TheWeek, 18 June 2026
Always follow the manufacturer’s instructions for safety and efficacy when using a household steam cleaner.
—Louise Parks, Martha Stewart, 17 June 2026
Disease-modifying therapies have demonstrated efficacy in early symptomatic disease.
—Elizabeth Bevins, STAT, 16 June 2026
See All Example Sentences for efficacy
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Ryan Brennan, Sacbee.com, 18 June 2026
Operating ‘beyond the bounds of FDA approval’ Although there is a lack of FDA approval and little evidence of its efficacy, stem cell treatments for autism are being steadily provided across the country.
—Theara Coleman, TheWeek, 18 June 2026
Always follow the manufacturer’s instructions for safety and efficacy when using a household steam cleaner.
—Louise Parks, Martha Stewart, 17 June 2026
Disease-modifying therapies have demonstrated efficacy in early symptomatic disease.
—Elizabeth Bevins, STAT, 16 June 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 21 Jun. 2026.
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