ef·​fi·​ca·​cious ˌe-fə-ˈkā-shəs How to pronounce efficacious (audio)
: having the power to produce a desired effect
an efficacious remedy
efficaciously adverb
efficaciousness noun

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Effective, effectual, and efficient are synonyms of efficacious, but each of these words has a slightly different connotation. Efficacious suggests possession of a special quality or virtue that makes it possible to achieve a result ("a detergent that is efficacious in removing grease"). Effective stresses the power to produce or the actual production of a particular effect ("an effective rebuttal"), while effectual suggests the accomplishment of a desired result, especially as viewed after the fact ("measures taken to reduce underage drinking have proved effectual"). The last synonym, efficient, implies an acting or potential for acting that avoids loss or waste of energy ("an efficient small car").

Choose the Right Synonym for efficacious

effective, effectual, efficient, efficacious mean producing or capable of producing a result.

effective stresses the actual production of or the power to produce an effect.

an effective rebuttal

effectual suggests the accomplishment of a desired result especially as viewed after the fact.

the measures to stop the pilfering proved effectual

efficient suggests an acting or a potential for action or use in such a way as to avoid loss or waste of energy in effecting, producing, or functioning.

an efficient small car

efficacious suggests possession of a special quality or virtue that gives effective power.

a detergent that is efficacious in removing grease

Examples of efficacious in a Sentence

taking a cookie break while studying is one of the most efficacious ways of rejuvenating the mind that I have ever discovered
Recent Examples on the Web With chic packaging, efficacious formulas, and clean ingredients like soothing tiger grass, hydrating allantoin, and rich kalahari melon and moringa, this balm is a jelly-to-oil-to-milk formula that can be used on both the face and eyes without irritation. Iman Balagam, Vogue, 25 Jan. 2024 Greenwood is hopeful that further progress will yield cheap vaccines that require fewer doses and remain efficacious for longer, enabling vaccination coverage to be widened and possibly even expanded to other parts of the world. David Cox, WIRED, 22 Jan. 2024 Health-conscious consumers are seeking voices of authority and efficacious products supported by scientific evidence. Jessica Mathews, Fortune, 5 Jan. 2024 Infused with efficacious plant oils for the ultimate ritual, the luxurious trio creates a deeply nourishing moment for the body. Angela Lei, Forbes, 29 Nov. 2023 Consider Medik8, the British brand whose efficacious skin solutions are now available stateside. Jenny Berg, Vogue, 27 Nov. 2023 For over 30 years, our editors have been swatching and swiping and slathering on thousands of beauty products to discover the most innovative, efficacious launches on the market. Allure Editors, Allure, 13 Sep. 2023 Serving the essential role of creating safe and efficacious medicines requires a financially viable pharmaceutical industry, which means that the profitability of the pharmaceutical industry must be competitive in the financial marketplace. Wayne Winegarden, Forbes, 17 July 2023 But when evaluating with the efficacy of masking, the Royal Society’s report leaned heavily on 46 observational studies that, in contrast to the RCT studies reviewed in the definitive Cochrane study, concluded that mask mandates were efficacious. Lars Jonung, National Review, 21 Sep. 2023 See More

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'efficacious.' 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 Middle French efficace "effective" or its source Latin efficāc-, efficāx "capable of fulfilling a function, effective" (from efficere "to make, bring about, produce, carry out" + -āc-, -āx, deverbal suffix denoting habitual or successful performance) + -ious — more at effect entry 1, audacious

First Known Use

1528, in the meaning defined above

Time Traveler
The first known use of efficacious was in 1528


Dictionary Entries Near efficacious

Cite this Entry

“Efficacious.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/efficacious. Accessed 21 Feb. 2024.

Kids Definition


ef·​fi·​ca·​cious ˌef-ə-ˈkā-shəs How to pronounce efficacious (audio)
: having the power to produce a desired result
an efficacious remedy
efficaciously adverb
efficaciousness noun

More from Merriam-Webster on efficacious

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