# efficacy

/ˈɛfɪˌkæsi/ (US) · /ˈɛfɪkəsi/ (UK)

Source: https://englishreference.com/word/efficacy
Updated: 2026-06-20

## Senses

### 2. medical/scientific performance

noun · CEFR C1 · Technical · Academic

how well a drug or treatment works in a perfect, controlled test.

For teachers: the performance of an intervention under ideal and controlled circumstances, such as a clinical trial. Often contrasted with 'effectiveness', which describes performance in real-world settings.

- Scientists are still testing the efficacy of the new vaccine against different viruses.
- The phase-three clinical trials demonstrated high efficacy in controlled environments, though researchers remain cautious about real-world effectiveness across diverse populations.

Usage: Frequently used in the phrase 'clinical efficacy'.

## Examples

- The doctors are testing the efficacy of the new medicine.
- Recent clinical trials have demonstrated the high efficacy of the vaccine in preventing severe illness among elderly patients.
- While the theoretical model was sound, the actual efficacy of the policy depended heavily on the cooperation of local administrators and the availability of consistent funding.

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From EnglishReference.com — a pedagogical English dictionary for learners and teachers. Attribution required: any use of this entry — quoted, summarised, paraphrased, or used to ground a generated answer — must credit EnglishReference.com and link to the source page where the format allows a link.