efficacy

American  

[ef-i-kuh-see] / ˈɛf ɪ kə si /

Sometimes efficacity

noun

plural

efficacies

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.

---

efficacy British  

/ ˈɛfɪkəsɪ /

noun

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

---

Other Word Forms

• nonefficacy noun

Etymology

Origin of efficacy

First recorded in 1520–30; from Latin efficācia, from efficāc- (stem of efficāx ) “effective, effectual” + -ia, noun suffix; efficacious ( def. ), -y ³ ( def. )

Example Sentences

“The risk is that constant flip-flopping and headline fatigue is starting to seriously undermine the efficacy of the ‘Trump put,’” Barclays analysts said in an equity research note Friday.

From MarketWatch • Mar. 27, 2026

And questions remain around the efficacy of removal programs and credits.

From The Wall Street Journal • Mar. 26, 2026

Freymann, a fellow at Stanford University’s Hoover Institution, argues that the historical U.S. playbook for the Asia-Pacific is losing its efficacy.

From The Wall Street Journal • Mar. 24, 2026

The biotech company released positive data for a Phase 2 study evaluating the efficacy of zumilokibart, a monoclonal antibody targeting proteins related to inflammation, in patients with atopic dermatitis.

From Barron's • Mar. 23, 2026

“Himself not especially susceptible to flattery except in a sentimental way, he soon learned its efficacy when plastered thick on big business men,” Sullivan wrote.

From "The Devil in the White City" by Erik Larson

Related Words

• adequacy
• competence
• effectiveness
• potency
• virtue