efficacious

C2+ Topic:Cause, Change & Process

US /ˌɛfɪˈkeɪʃəs/ UK /ˌɛfɪˈkeɪʃəs/ See IPA manual

adj

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... effective effectual potent successful productive powerful

Opposites

ineffective inefficacious futile useless

Word family

Derived forms efficaciouslyefficaciousness

Related forms efficacy efficacity effect

Etymology

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

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Topics

Cause, Change & Process Word Formation & Derivatives

# 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

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