efficacy

American  

[ef-i-kuh-see] / ˈɛf ɪ kə si /

Sometimes efficacity

noun

• efficacies
  plural

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

Derived Forms

• nonefficacy
  noun

Inflected Forms

Nouns

• efficacies
  plural

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

See Examples For:

efficacy efficacies

“Another thing about emphasizing the efficacy of youthful endeavor in changing the world is the riff of the song was written by Roman,” Morello explained.

From Salon ● Sep. 22, 2026

At the same time, he is ambivalent about the efficacy of outright bans on school devices.

From Los Angeles Times ● Sep. 14, 2026

These breaks are a welcome respite from long stretches of sitting and screen-based learning, which is common in some classrooms despite questions about its efficacy.

From Slate ● Sep. 5, 2026

Conscious feelings have more efficacy than nonconscious processes, meaning that once they emerged, they conferred so great an advantage that natural selection had to favor them.

From The Wall Street Journal ● Sep. 4, 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

For the next pandemic, if we have certain vaccines with starting efficacies, we can predict the minimum proportion we need to vaccinate to achieve vaccine-induced herd immunity.

From Scientific American ● Apr. 6, 2023

That would make it still well above the 50% threshold needed for FDA authorization, but would pale in comparison to the 90-plus efficacies of other COVID-19 vaccines.

From Science Magazine ● Mar. 25, 2021

Further tests would be needed to judge how much the 2P mutation contributes to the overall efficacies of the frontrunner vaccines.

From National Geographic ● Dec. 31, 2020

Obsessed with holistic health, we are also susceptible to pitches about the mysterious efficacies of neurons and transmitters.

From Slate ● Jan. 7, 2014

The benignant efficacies of Concealment," cries our Professor, "who shall speak or sing?

From Sartor Resartus: the life and opinions of Herr Teufelsdröckh by Thomas Carlyle

Related Words

• adequacy
• competence
• effectiveness
• potency
• virtue