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.

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efficacy British  

/ ˈɛfɪkəsɪ /

noun

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

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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

"Some of the SPF testing, I feel, has become a bit more of a marketing exercise than a real reflection of efficacy," cosmetic chemist Michelle Wong told the BBC last year.

From BBC • Mar. 26, 2026

Analysts at the investment bank acknowledge that therapy efficacy data is encouraging but stress that there will be no near-term material impact on the Australia-listed hearing-implant maker.

From The Wall Street Journal • Mar. 25, 2026

ImmunityBio received an FDA warning for misrepresenting Anktiva’s efficacy and administration, violating federal law.

From Barron's • Mar. 24, 2026

Three were scientists checking the efficacy of sprays; 8 worked in greenhouses; 5 were farm workers.

From "Silent Spring" by Rachel Carson

Related Words

• adequacy
• competence
• effectiveness
• potency
• virtue