efficacious

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

• Rhymes: -eɪʃəs

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.

• effect
• effective
• effectivity
• effectual
• efficacity
• efficacy
• efficiency
• efficient

• efficacious in Webster’s Revised Unabridged Dictionary, G. & C. Merriam, 1913.
• efficacious in The Century Dictionary, New York, N.Y.: The Century Co., 1911.
• “efficacious” in Lexico, Dictionary.com; Oxford University Press.