efficacious

/ˌefɪˈkeɪʃəs/ (bre, ipa) · /ˌefɪˈkeɪʃəs/ (ame, ipa) · /ˌe-fə-ˈkā-shəs/ (ame, mw)

efficacious — adjective

  • efficaciouspositive
  • more efficaciouscomparative
  • most efficacioussuperlative

1. producing the result or effect that you want

1.形容詞C1
釋義

producing the result or effect that you want

例句

Ananya found the new teaching method highly efficacious with her struggling students.

collocation: highly efficacious + with + noun phrase

The lawyer's efficacious argument persuaded every member of the jury.

同義詞
  • effective

    much more common and less formal; the everyday word for 'works well'

  • potent

    emphasises strength or power rather than simply achieving a result

  • effectual

    similarly formal but rarer; stresses that the result was actually achieved

  • fruitful

    positive in tone; used mainly of efforts, discussions, or collaborations that yield good outcomes

反義詞
  • ineffective

    the direct opposite; fails to produce the wanted result

  • futile

    stronger — suggests the effort itself is pointless

用法筆記

This is a formal word. In everyday speech and writing, "effective" is far more common and natural. Use "efficacious" in formal contexts such as academic papers, legal documents, or when you want to emphasise proven, measurable results.

常見錯誤

❌The new system is more efficacious than the old one because it uses less energy.
✅The new system is more efficient than the old one because it uses less energy.
💡'efficacious' means producing results; 'efficient' means producing results with minimal waste of time, energy, or resources.
❌I took some efficacious cough medicine and felt better the next day.' (in casual conversation)
✅I took some effective cough medicine and felt better the next day.
💡in everyday speech, 'effective' sounds natural; 'efficacious' sounds overly formal.

2. describes a medicine, treatment, or therapy that works well under ideal, careful

2.形容詞C1
釋義

describes a medicine, treatment, or therapy that works well under ideal, carefully managed test conditions

例句

Dr. Fatima's team reported that the new vaccine was efficacious in phase-three clinical trials.

medical: efficacious in [phase] clinical trials

The herbal remedy proved efficacious under laboratory conditions but less so at home.

contrast: efficacious under laboratory conditions vs. real-world use

同義詞
  • clinically effective

    more common phrase in medical English; less formal-sounding

  • therapeutic

    broader — refers to healing generally, not necessarily proven in trials

反義詞

用法筆記

Primarily used in medical and scientific writing. This sense is narrower than sense 1 (PRODUCING RESULTS): it refers specifically to results shown under controlled experimental conditions, not in everyday real-world use. In medical research, "efficacious" (works in trials) is often contrasted with "effective" (works in ordinary clinical practice).

常見錯誤

❌The medicine was efficacious for my headache.' (without trial context)
✅The medicine was effective for my headache.
💡sense 2 refers to controlled trial results, not personal anecdotal experience.