[ ef-i-kuh-see ]
See synonyms for: efficacyefficacies on Thesaurus.com

noun,plural ef·fi·ca·cies.
  1. the capacity for producing a desired result or effect: Short, frequent periods of practice were shown to have greater efficacy than longer and less frequent ones.

  2. a measure of the success of a vaccine or other pharmaceutical when used in the controlled environment of a clinical trial, as opposed to in the real world.: Compare effectiveness (def. 3).

Origin of efficacy

First recorded in 1520–30; from Latin efficācia, from efficāc- (stem of efficāx ) “effective, effectual” + -ia, noun suffix; see efficacious, -y3
  • Sometimes ef·fi·cac·i·ty [ef-i-kas-i-tee] /ˌɛf ɪˈkæs ɪ ti/ .

Other words from efficacy

  • non·ef·fi·ca·cy, noun

Words Nearby efficacy

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

How to use efficacy in a sentence

British Dictionary definitions for efficacy


/ (ˈɛfɪkəsɪ) /

  1. the quality of being successful in producing an intended result; effectiveness

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012