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efficacy

EF-ih-kuh-see /ˈefɪkəsi/

nounC1GRESATTOEFLUPSC

academicscientificprofessional

The ability to produce a desired or intended result.

Efficacy is highly objective and measurable. It is used extensively in medicine, public policy, and organizational management to describe whether a specific intervention, drug, or strategy actually works under controlled conditions.

The story behind the word. Efficacy traces back to the Latin efficere, which means 'to work out' or 'bring to pass'. It is built from ex- (out) and facere (to make). Literally, efficacy is the power to 'make something come out' of your efforts, turning a plan or a drug into a tangible, real-world result.

Word relationships

effectiveness: Effectiveness is a general term for producing a result in real-world conditions, whereas efficacy is often used in scientific or formal contexts to describe the inherent power to produce that result under ideal, controlled conditions.
potency: Potency refers specifically to the strength or power of something (like a drug or an argument), while efficacy refers to its ability to achieve the specific intended outcome.

inefficacy: Inefficacy is the complete failure or inability of a treatment or strategy to produce any desired effect.
futility: Futility is the state of being completely useless or pointless, where no effort can produce a result.

Commonly confused with

efficiency: Efficiency is about minimizing waste (time, money, effort) while performing a task, whereas efficacy is strictly about whether the task successfully achieves its goal.

Word family

efficacious (adjective): Successful in producing a desired or intended result; effective.
efficaciously (adverb): In a manner that successfully produces the desired result.

Collocations

demonstrated efficacy
self-efficacy
therapeutic efficacy
test the efficacy

Idioms & expressions

do the trick
produce the goods

Example sentences

"The clinical trials demonstrated that the new vaccine had an efficacy rate of over ninety-five percent." A medical journal discussing the results of a clinical trial.
"We must believe in the efficacy of our own efforts if we are to achieve any lasting social reform." A philosophical discussion on the power of human action.

Memory hook

Efficacy sounds like 'EFFECT-I-CAN-SEE'. If a medicine or strategy has efficacy, it produces an effect that you can clearly see. Efficacy = Effect I can see.

When not to use

Do not confuse 'efficacy' with 'efficiency.' Efficiency is about doing things with the least waste of time and effort; efficacy is simply about whether the desired result is achieved at all.

Fun facts

In psychology, 'self-efficacy' is a famous concept coined by Albert Bandura, referring to an individual's belief in their own capability to execute behaviors necessary to produce specific performance attainments.