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
The vibes and junior hockey tactics that functioned early in his tenure seem to have lost all efficacy.
—Andrew Knoll, Daily News, 7 Feb. 2026
The efficacy of pre-treating roads with salt can also be limited by how much wintry precipitation falls.
—Julia James, Dallas Morning News, 6 Feb. 2026
The experimental drug drove solid weight loss when taken once a month in a mid-stage trial – offering early evidence that the injection can be administered less frequently than existing drugs without sacrificing efficacy.
—Annika Kim Constantino, CNBC, 6 Feb. 2026
Lowering standards for safety and efficacy for people with obesity reinforces harmful inequities in care.
—Emily Kay Votruba, EverydayHealth.com, 6 Feb. 2026
See All Example Sentences for efficacy
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Andrew Knoll, Daily News, 7 Feb. 2026
The efficacy of pre-treating roads with salt can also be limited by how much wintry precipitation falls.
—Julia James, Dallas Morning News, 6 Feb. 2026
The experimental drug drove solid weight loss when taken once a month in a mid-stage trial – offering early evidence that the injection can be administered less frequently than existing drugs without sacrificing efficacy.
—Annika Kim Constantino, CNBC, 6 Feb. 2026
Lowering standards for safety and efficacy for people with obesity reinforces harmful inequities in care.
—Emily Kay Votruba, EverydayHealth.com, 6 Feb. 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 8 Feb. 2026.
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