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; see efficacious ( def. ), -y ³ ( def. )

Explanation

The degree to which a method or medicine brings about a specific result is its efficacy. You might not like to eat it, but you can't question the efficacy of broccoli as a health benefit. Efficacy is a more formal way to say effectiveness, both of which stem from the Latin verb efficere "to work out, accomplish." The effectiveness, or efficacy, of something is how well it works or brings the results you hoped for. A scientist does research to determine the efficacy of a vaccine or medicine under development. If it is efficacious, it will cure or prevent a disease.

Example Sentences

After reports that vitamin K deficiency bleeding was on the rise, the American Academy of Pediatrics updated its policy statement in 2022 to stress the shot’s safety and efficacy.

From Salon • May 7, 2026

The problem with this statement is that it ignores a long history demonstrating the drug’s safety and efficacy.

From Slate • May 4, 2026

“Our confidence in the safety and efficacy of Foundayo is based on our robust clinical trial program, which followed over 11,000 participants for up to two years,” the Lilly spokesperson told Barron’s.

From Barron's • May 4, 2026

"Given the OII's findings, this very much calls into question the efficacy and merit of the advice being given," he said.

From BBC • Apr. 29, 2026

But there’ll always be some people who will do extremely well, and they will loudly swear to the efficacy of whatever system they’ve used.

From "Innumeracy: Mathematical Illiteracy and Its Consequences" by John Allen Paulos

Related Words

• adequacy
• competence
• effectiveness
• potency
• virtue