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
According to the Lilly statement, this discovery is concerning because nothing is known about the short- or long-term effects of this molecule in humans, or whether there are any potential impacts on the drug’s efficacy, toxicity, or immune reactions.
—Tom Gavin, EverydayHealth.com, 13 Mar. 2026
The new details related to the Chinese and Iranian accounts, however, raise new questions about the efficacy of that program.
—Leo Schwartz, Fortune, 12 Mar. 2026
The telehealth companies that have been warned — with names like Lovely Meds, Hello Cake, and MEDVi — don’t directly prescribe the medications, which are not approved by the FDA or evaluated for safety and efficacy.
—Katie Palmer, STAT, 12 Mar. 2026
Despite its intense efficacy, the body treatment is gentle and safe for eczema-prone complexions and all skin types.
—Kyra Surgent, InStyle, 11 Mar. 2026
See All Example Sentences for efficacy
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Tom Gavin, EverydayHealth.com, 13 Mar. 2026
The new details related to the Chinese and Iranian accounts, however, raise new questions about the efficacy of that program.
—Leo Schwartz, Fortune, 12 Mar. 2026
The telehealth companies that have been warned — with names like Lovely Meds, Hello Cake, and MEDVi — don’t directly prescribe the medications, which are not approved by the FDA or evaluated for safety and efficacy.
—Katie Palmer, STAT, 12 Mar. 2026
Despite its intense efficacy, the body treatment is gentle and safe for eczema-prone complexions and all skin types.
—Kyra Surgent, InStyle, 11 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 16 Mar. 2026.
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