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

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

From Barron's • Mar. 24, 2026

The companies said a Phase 3 study of its Lyme disease vaccine candidate demonstrated more than 70% efficacy in preventing Lyme disease in those 5 and older.

From MarketWatch • Mar. 23, 2026

Muscle growth and exploratory functional efficacy weren’t consistent nor robust enough to move the drug to late-stage development, the company said.

From The Wall Street Journal • Mar. 23, 2026

"This is a pilot across Scotland to help inform the UK screening committee about its efficacy and to allow them to take a further recommendation about its wider rollout."

From BBC • Mar. 22, 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