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[ef-i-key-shuh s] /ˌɛf ɪˈkeɪ ʃəs/
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 forms
efficaciously, adverb
efficaciousness, noun
nonefficacious, adjective
nonefficaciously, adverb
unefficacious, adjective
unefficaciously, adverb Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2015.
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Examples from the web for efficacious
  • We have found that in most of the common cases a homeopathic remedy is frequently efficacious.
  • More efficacious ways of implementing regulation would help solve this problem.
  • Until they are implemented on a suitable scale and found to be efficacious and sustainable, they remain just ideas.
  • The competition has certainly proved efficacious.
  • Even though we have medications that are efficacious, they're not being used.
  • It is the efficacious use of technology that may enhance teaching not the technology.
  • He points out that conventional medicines must be shown to be both safe and efficacious before they can be licensed for sale.
  • They point out that conventional medicines must be shown to be both safe and efficacious before they can be licensed for sale.
  • Only for adult postoperative and chemotherapy nausea and vomiting did they find clear evidence that acupuncture was efficacious.
  • And the new software happened to be far more efficacious for transmitting video files.
British Dictionary definitions for efficacious


capable of or successful in producing an intended result; effective as a means, remedy, etc
Derived Forms
efficaciously, adverb
efficaciousness, noun
Word Origin
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
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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
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