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 Centers for Disease Control and Prevention (CDC), some studies report that certain essential oils, like peppermint oil and rosemary oil, have some efficacy for tick control.
—Dan Simms, USA Today, 4 June 2026
Oncologists want to know if that broad activity leads to broad efficacy.
—Lisa Jarvis, Mercury News, 4 June 2026
Novo shares are still down about 15% year-to-date, mostly due to double-digit drops following a gloomy guidance and a trial which showed its next-generation weight-loss drug CagriSema failed to match Lilly’s Zepbound’s efficacy.
—Elsa Ohlen, CNBC, 3 June 2026
Safety and efficacy have not been evaluated in pregnant women, pediatric users, people with active cancer or cancer in remission, or users with uncontrolled hypertension, hypotension, bradycardia or tachycardia.
—Samantha Agate, Sacbee.com, 3 June 2026
See All Example Sentences for efficacy
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Dan Simms, USA Today, 4 June 2026
Oncologists want to know if that broad activity leads to broad efficacy.
—Lisa Jarvis, Mercury News, 4 June 2026
Novo shares are still down about 15% year-to-date, mostly due to double-digit drops following a gloomy guidance and a trial which showed its next-generation weight-loss drug CagriSema failed to match Lilly’s Zepbound’s efficacy.
—Elsa Ohlen, CNBC, 3 June 2026
Safety and efficacy have not been evaluated in pregnant women, pediatric users, people with active cancer or cancer in remission, or users with uncontrolled hypertension, hypotension, bradycardia or tachycardia.
—Samantha Agate, Sacbee.com, 3 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 7 Jun. 2026.
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