efficacy

noun

ef·​fi·​ca·​cy ˈe-fi-kə-sē How to pronounce efficacy (audio)
plural efficacies
Synonyms of efficacy
: 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

Synonyms of efficacy

Relevance
See All Synonyms & Antonyms in Thesaurus

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
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.
Lack of self-efficacy Lahey explains that kids who never have to deal with the consequences of their actions end up feeling a lack of self-efficacy. — Cheryl Maguire, Parents, 1 Sep. 2026 The need-to-know this morning Alumis said its experimental treatment for lupus failed to achieve the efficacy goals of a mid-stage clinical trial. — Helen Branswell, STAT, 1 Sep. 2026 The protein building blocks can have anti-inflammatory and muscle-building properties – though studies and safety analyses of their benefits and efficacy as supplements or injections vary widely. — Cara Kelly, USA Today, 30 Aug. 2026 These second-generation antihistamines have similar efficacy, fewer undesirable side effects, and far less potential for abuse, according to the FDA. — Dr. Jill Grimes, CNN Money, 29 Aug. 2026 See All Example Sentences for efficacy

Word History

Etymology

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

First Known Use

1527, in the meaning defined above

Time Traveler
The first known use of efficacy was in 1527

Rhymes for efficacy

See All Rhymes for efficacy

Browse Nearby Words

efficacity
efficacy
efficience
See all Nearby Words

Cite this Entry

“Efficacy.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/efficacy. Accessed 4 Sep. 2026.

Copy Citation

Kids Definition

efficacy

noun
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

Britannica.com: Encyclopedia article about efficacy

Last Updated: - Updated example sentences
Love words? Need even more definitions?

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!

More from Merriam-Webster

Word of the Day

Listen to the pronunciation of apiary

See Definitions and Examples »

Popular in Grammar & Usage

See More

A Who’s Who Regarding ‘Who’s’ and ‘Whose’

A List of Most Commonly Confused Words

Why do we use apostrophes to show possession?

Is it 'nerve-racking' or 'nerve-wracking'?

What's the difference between 'fascism' and 'socialism'?
See More

Popular in Wordplay

See More

The Longest Long Words List

Stay Woke

Ye Olde Nincompoop: Old-Fashioned Words for 'Stupid'

14 Phobias You Probably Haven't Heard Of

The Words of the Week - Sept. 4
See More

Popular

See More

A Who’s Who Regarding ‘Who’s’ and ‘Whose’

The Longest Long Words List

Stay Woke
See More

Games & Quizzes

See All
Play Quordle: Guess all four words in a limited number of tries. Each of your guesses must be a real 5-letter word.
Quordle Can you solve 4 words at once?
Play Blossom: Solve today's spelling word game by finding as many words as you can using just 7 letters. Longer words score more points.
Blossom Pick the best words!
Play Missing Letter: a crossword with a twist. Each of the 25 puzzle words start with a different letter of the alphabet. Which letter is missing?
The Missing Letter A daily crossword with a twist
anagrams on a chalk board
Word Builder Quiz Vol. 2 Pick the letter that when added to a word will ma...
See All