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From Old French efficacieux, from Latin efficāx (efficacious) +‎ -ous, from efficere (to effect, to accomplish); see effect.



efficacious (comparative more efficacious, superlative most efficacious)

  1. (formal) Effective; possessing efficacy. [from 1520s]
    this medicine is efficacious
    Synonym: effective
    Antonym: inefficacious
    • 1969, Susan Sontag, “What’s Happening in America”, in Styles of Radical Will, Kindle edition, Penguin Modern Classics, published 2009, →ISBN, page 195:
      The unquenchable American moralism and the American faith in violence are not just twin symptoms of some character neurosis taking the form of a protracted adolescence, which presages an eventual maturity. They constitute a full-grown, firmly installed national psychosis, founded, as are all psychoses, on the efficacious denial of reality.

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