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

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

While the Dalai Lama expressed sorrow and questioned the efficacy of such protests, some scholars criticized him for not trying to stop the practice.

From Barron's ● Aug. 7, 2026

A spokesman added: "All pet parasite treatments undergo rigorous scientific assessment by independent regulatory authorities before approval for use, and their safety and efficacy continue to be monitored."

From BBC ● Aug. 5, 2026

Panel member Elizabeth Rebello, a pharmacist and anaesthesiologist, told the committee she had voted no on BPC-157 "because of the lack of efficacy data" and "safety concerns", Reuters news agency reported.

From BBC ● Jul. 24, 2026

The drugmaker said Friday that efficacy and safety data generated to date doesn’t support further development of the drug, amlitelimab, in atopic dermatitis.

From The Wall Street Journal ● Jul. 24, 2026

In this domain, the prize for efficacy goes to the Cantonese dialect of Chinese, whose brevity grants residents of Hong Kong a rocketing memory span of about 10 digits.

From "Outliers" by Malcolm Gladwell

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

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

Along these lines, computational studies have shown that ligands with different efficacies modulate the free-energy landscape of the receptor by shifting the conformational equilibrium towards active or inactive conformations depending on their elicited physiological response.

From Nature ● Feb. 13, 2013

He will inquire of art,—the empiric art,—and rude accident, what latent efficacies they have detected in her, what churlish secrets of hers they have wrung from her.

From The Philosophy of the Plays of Shakspere Unfolded by Delia Bacon

Related Words

• adequacy
• competence
• effectiveness
• potency
• virtue