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.

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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

Derived Forms

• nonefficacy
  noun

Inflected Forms

Nouns

• efficacies
  plural

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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. )

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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

Trying to explain the device, and how her claims of its efficacy were misread by investors and the public, Holmes once again can’t translate her thoughts into words that make sense to Fielder.

From Salon ● Oct. 17, 2026

"This lets us speak about safety and efficacy with real confidence," Dr. Fleckenstein said.

From Science Daily ● Oct. 14, 2026

This suggests petrelintide’s weight-loss efficacy fell by between 15% and 20% for this group compared with nondiabetic obesity patients, for whom Zealand previously reported a 10.7% reduction, the analysts say.

From The Wall Street Journal ● Oct. 9, 2026

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

From Los Angeles Times ● Sep. 14, 2026

But questions about the painlessness and efficacy of lethal injection were emerging.

From "Just Mercy" by Bryan Stevenson

To get a little technical, the Pfizer data, in particular, had wide confidence intervals—basically, the efficacies are more like pretty good estimates, than numbers that the scientists are really certain of.

From Slate ● Jun. 22, 2022

For young kids, there are now two vaccines available with very similar mechanisms but different doses, dosing intervals and efficacies.

From Scientific American ● Jun. 15, 2022

Still, the vaccines, shown in clinical trials to have efficacies of up to 95% against symptomatic disease, have finally given the world the prospect of an escape from COVID-19’s long siege.

From Science Magazine ● Feb. 16, 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

In the face of the Relief Pills exposure, Hal could no longer excuse his father on the ground that Dr. Surtaine honestly credited his medicines with impossible efficacies.

From The Clarion by William Dodge Stevens

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Related Words

• adequacy
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• effectiveness
• potency
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