[ef-i-key-shuh s]


capable of having the desired result or effect; effective as a means, measure, remedy, etc.: The medicine is efficacious in stopping a cough.

Origin of efficacious

1520–30; < Latin efficāci- (stem of efficāx) effectual. See efficient, -acious
Related formsef·fi·ca·cious·ly, adverbef·fi·ca·cious·ness, nounnon·ef·fi·ca·cious, adjectivenon·ef·fi·ca·cious·ly, adverbun·ef·fi·ca·cious, adjectiveun·ef·fi·ca·cious·ly, adverb
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for efficacious

Contemporary Examples of efficacious

  • I asked him if he thought cattle cars would be an efficacious means of transporting the Palestinians to their new homes.

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    The Wildest Moments From CPAC

    Max Blumenthal

    March 2, 2009

Historical Examples of efficacious

British Dictionary definitions for efficacious



capable of or successful in producing an intended result; effective as a means, remedy, etc
Derived Formsefficaciously, adverbefficaciousness, noun

Word Origin for efficacious

C16: from Latin efficāx powerful, efficient, from efficere to achieve; see effect
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for efficacious

"sure to have the desired effect" (often of medicines), 1520s, from Latin efficaci-, stem of efficax (see efficacy) + -ous. Related: Efficaciously; efficaciousness.

Online Etymology Dictionary, © 2010 Douglas Harper