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
Harvard Health has taken a critical look at Apollo Neuro, and Medscape has warned about uncertain efficacy in at-home vagus stimulation devices.
—
Samantha Agate,
Charlotte Observer,
3 July 2026
The first in-human clinical trial will assess the drug's safety profile, hopefully providing a green light for further efficacy studies.
—
New Atlas,
2 July 2026
Belief in the efficacy of bloodletting was well-established and undisputed until well into the 1800s.
—
Katherine Ott,
The Conversation,
2 July 2026
Harvard Health has published a critical take on Apollo Neuro, and Medscape has cautioned about at-home vagus nerve stimulation devices, warning of uncertain efficacy and safety.
—
Samantha Agate,
Kansas City Star,
2 July 2026
See All Example Sentences for efficacy
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Samantha Agate,
Charlotte Observer,
3 July 2026
The first in-human clinical trial will assess the drug's safety profile, hopefully providing a green light for further efficacy studies.
—
New Atlas,
2 July 2026
Belief in the efficacy of bloodletting was well-established and undisputed until well into the 1800s.
—
Katherine Ott,
The Conversation,
2 July 2026
Harvard Health has published a critical take on Apollo Neuro, and Medscape has cautioned about at-home vagus nerve stimulation devices, warning of uncertain efficacy and safety.
—
Samantha Agate,
Kansas City Star,
2 July 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 5 Jul. 2026.
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