efficacious C2+ Topic:Cause, Change & Process US /ˌɛfɪˈkeɪʃəs/ UK /ˌɛfɪˈkeɪʃəs/ See IPA manual Meanings 1 adj effective in producing a desired result The new vaccine proved highly efficacious against the strain. Thesaurus Synonyms 1 adj · effective in producing a... effectiveeffectualpotentsuccessfulproductivepowerful ↔ Opposites ineffectiveinefficaciousfutileuseless Word family Derived forms efficaciouslyefficaciousness Related forms efficacyefficacityeffect Etymology AI details From Latin efficāx (genitive efficācis, 'effective, powerful'), from efficere ('to accomplish'), + the English suffix -ous; from ex- ('out') + facere ('to make'). efficāx Latin efficere Latin facere Latin View etymology graph -> Topics Cause, Change & Process Word Formation & Derivatives External links Cambridge Dictionary Wiktionary Google Images # efficacious **Type:** Word **Level:** C2+ **Part of speech:** adj **IPA (US):** /ˌɛfɪˈkeɪʃəs/ **IPA (UK):** /ˌɛfɪˈkeɪʃəs/ ## Meanings 1. effective in producing a desired result ## Examples - The new vaccine proved highly efficacious against the strain. ## Synonyms - effective - effectual - potent - successful - productive - powerful ## Antonyms - ineffective - inefficacious - futile - useless ## Derived Forms - efficaciously - efficaciousness ## Related Forms - efficacy - efficacity - effect ## Roots and Affixes - Root: efficāx - Root: efficere - Root: facere ## Etymology From Latin efficāx (genitive efficācis, 'effective, powerful'), from efficere ('to accomplish'), + the English suffix -ous; from ex- ('out') + facere ('to make'). ## AI Etymology Efficacious is from Latin efficāx (genitive efficācis, 'effective, powerful'), from efficere ('to accomplish'), + the English suffix -ous; from ex- ('out') + facere ('to make'). In present-day use, it means effective in producing a desired result. Derived formations include efficaciously and efficaciousness. Related vocabulary includes efficacy, efficacity, and effect. ## Topics - Cause, Change & Process - Word Formation & Derivatives markdown