English Reference
noun

efficacy

n.
US /ˈɛfɪˌkæsi/ UK /ˈɛfɪkəsi/ ef·fi·ca·cy UK spelling efficacies

  1. 01 C1 Advanced TechnicalAcademic Medicine

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

    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.

    Simple

    Scientists are still testing the efficacy of the new vaccine against different viruses.

    In context

    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'.

Within the current work, the research team closely investigated eugenol, clove oil's dominant bioactive component, to understand its action and further assess its safety and efficacy.

Phys.org· 15 Aug 2026

This provides information about efficacy and can reveal patterns, such as when symptoms are most severe and if they change month to month.

The Guardian· 14 Aug 2026

From Old French efficace, from Late Latin efficācia (“efficacy”), from efficāx (“efficacious”); see efficacious.

Etymology adapted from Wiktionary, available under CC BY-SA 4.0.

This entry
Origin la-lat efficācia
Level C1 · Advanced
Frequency Frequent
Updated