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 bottom line Jüced is made with ingredients that have research to support their efficacy at increasing energy levels and endurance, particularly caffeine.
—Abbey Stone, Bon Appetit Magazine, 19 Dec. 2025
Major health authorities like the World Health Organization have affirmed the safety and efficacy of the COVID-19 vaccines, which researchers estimate prevented more than 14 million deaths worldwide in their first year.
—Jessica Schreifels, CNN Money, 18 Dec. 2025
Reed's latest attack and arrest have raised questions about the efficacy of electronic monitoring.
—Adam Harrington, CBS News, 18 Dec. 2025
Always wear sunscreen, but read labels carefully, as many can be misleading about safety and efficacy.
—Nicole Fallert, USA Today, 16 Dec. 2025
See All Example Sentences for efficacy
Examples are automatically compiled from online sources to
show current usage.
Read More
Opinions expressed in the examples do not represent those of Merriam-Webster or its editors.
Send us feedback.
Abbey Stone, Bon Appetit Magazine, 19 Dec. 2025
Major health authorities like the World Health Organization have affirmed the safety and efficacy of the COVID-19 vaccines, which researchers estimate prevented more than 14 million deaths worldwide in their first year.
—Jessica Schreifels, CNN Money, 18 Dec. 2025
Reed's latest attack and arrest have raised questions about the efficacy of electronic monitoring.
—Adam Harrington, CBS News, 18 Dec. 2025
Always wear sunscreen, but read labels carefully, as many can be misleading about safety and efficacy.
—Nicole Fallert, USA Today, 16 Dec. 2025
See All Example Sentences for efficacy
Cite this Entry
“Efficacy.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/efficacy. Accessed 26 Dec. 2025.
Copy CitationKids Definition
Love words? Need even more definitions?
Merriam-Webster unabridged
Share