ef·​fi·​ca·​cy ˈe-fi-kə-sē How to pronounce efficacy (audio)
plural efficacies
: the power to produce an effect

Examples 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
Recent Examples on the Web In it, the safety and efficacy of a twice-yearly, under-the-skin (subcutaneous) injection of lenacapavir for PrEP was compared with once-daily oral Descovy or Truvada in more than 5,300 cisgender women and adolescent girls aged 16 to 25 across 25 sites in South Africa and Uganda. New Atlas, 15 July 2024 Bloomberg News Generic versions of erectile dysfunction drugs Viagra and Cialis, among other medications, were allowed on the U.S. market using potentially problematic data that call into question their safety and efficacy, a Bloomberg analysis found. Tribune News Service, The Mercury News, 10 July 2024 At the trial's highest dose, the treatment showed some efficacy against depression in patients who had previously tried an average of nearly five antidepressants without success, according to the results published Monday in Nature Medicine. Beth Mole, Ars Technica, 25 June 2024 Gilead Sciences emerged as the week’s top-performing stock within the S&P 500 following the release of efficacy data for its potential HIV prevention drug, lenacapavir. Detroit Free Press, 22 June 2024 See all Example Sentences for efficacy 

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'efficacy.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History


borrowed from Latin efficācia, from efficāc-, efficāx "capable of fulfilling a function, efficacious" + -ia -y entry 2

First Known Use

13th century, in the meaning defined above

Time Traveler
The first known use of efficacy was in the 13th century

Dictionary Entries Near efficacy

Cite this Entry

“Efficacy.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/efficacy. Accessed 19 Jul. 2024.

Kids Definition


ef·​fi·​ca·​cy ˈef-i-kə-sē How to pronounce efficacy (audio)
plural efficacies
: the power to produce a desired result

More from Merriam-Webster on efficacy

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