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[ef-i-kuh-see] /ˈɛf ɪ kə si/
noun, plural efficacies.
capacity for producing a desired result or effect; effectiveness:
a remedy of great efficacy.
Origin of efficacy
1520-30; < Latin efficācia, equivalent to efficāc- (see efficacious) + -ia -y3
Related forms
nonefficacy, noun Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2015.
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Examples from the web for efficacy
  • The efficacy of research is by no means to be demeaned.
  • Some dermatologists have raised questions about the efficacy of the products and their claims.
  • The manufacturer also touts its efficacy among the hearing impaired or very old.
  • As life could not exist without service, neither can life survive without the efficacy of a beneficial service.
  • They acknowledged their own grave doubts as to the efficacy of such laws.
  • To prove their efficacy, the author supplies anecdotes detailing outrageous (and sometimes unintended) results.
  • I've never used it, so I can't comment on it's efficacy.
  • Her fame made her the emblem of a vigorous debate about the efficacy and legitimacy of so-called psychic detectives.
  • If somebody tends to be relatively negative, it affects their feelings of self-efficacy and likelihood of success.
  • His folksy manner masked a keen legal mind, a fiercely competitive streak and a relentless faith in the efficacy of punishment.
British Dictionary definitions for efficacy


the quality of being successful in producing an intended result; effectiveness
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
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Word Origin and History for efficacy

1520s, from Latin efficacia "efficacy, efficiency," from efficax (genitive efficacis) "powerful, effective," from stem of efficere "work out, accomplish" (see effect). Earlier in same sense was efficace (c.1200), from Old French eficace (14c.), from Latin efficacia; also efficacite (early 15c.), from Latin efficacitatem.

Online Etymology Dictionary, © 2010 Douglas Harper
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