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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]
    Synonym: effective
    Antonym: inefficacious
    This medicine is efficacious.
    • 1669 August 26, “[An Accompt of Some Books.] III. Ottonis Tachenii Hippocrates Chymicus. Venetiis in 12º.”, in Philosophical Transactions: Giving Some Accompt of the Present Undertakings, Studies and Labours of the Ingenious in Many Considerable Parts of the World, volume IV, number 50, London: Printed by T. N. for John Martyn [] , printer to the Royal Society, published 1670, →OCLC, pages 1019–1020:
      For, that the ordure, which continually gathers on the skin, would ſoon ſtop the pores of it, if the ſweat were not furniſht with ſome efficacious diſſolvent to open and pierce them.
    • 1834, L[etitia] E[lizabeth] L[andon], chapter XXXV, in Francesca Carrara. [], volume III, London: Richard Bentley, [], (successor to Henry Colburn), →OCLC, page 292:
      "I see," said he, with a smile, "that you, like myself, are trying the effect of this sweet evening for the headach. I have already found it very efficacious, and so, I think, have you,"—again smiling, as he noticed the deep blush which his sudden appearance had produced.
    • 1898, H.G. Wells, The War of the Worlds, London: William Heinemann, page 218:
      He would weep for hours together, and I verily believe that to the very end this spoilt child of life thought his weak tears in some way efficacious.
    • 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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