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



efficacy (usually uncountable, plural efficacies)

  1. Ability to produce a desired effect under ideal testing conditions.
    • 1642, Sir Thomas Browne, “Religio Medici [The Religion of a Doctor]”, in The Works Of the Learned Sr Thomas Brown, Kt., London: Tho. Basset et al., published 1686, page 15:
      [] and this hath even made me ſuſpect the efficacy of reliques, to examine the bones, queſtion the habits and appurtenances of Saints, and even of Chriſt himſelf.
    • 2005, Flay et al. Standards of Evidence: Criteria for Efficacy, Effectiveness and Dissemination DOI: 10.1007/s11121-005-5553-y
      Efficacy refers to the beneficial effects of a program or policy under optimal conditions of delivery, whereas effectiveness refers to effects of a program or policy under more real-world conditions.
  2. Degree of ability to produce a desired effect.
    • 1996, Moskovich, Patent application PCT/US1996/003658
      Toothbrush with improved efficacy
    • 2016, Wikipedia entry for "vaccine efficacy", as of 2016
      Vaccine efficacy is the percentage reduction of disease in a vaccinated group of people compared to an unvaccinated group, using the most favorable conditions.


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