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efficacy

The ability to produce a desired or intended result; effectiveness.

/ˈɛf.ɪ.kə.si/

[U] Uncountable
pl: efficacies

Used to describe the degree of power or capacity a thing has to produce an effect, such as measuring the efficacy of a drug.

💬Casual Conversation

🎬Tuesday afternoon, Chloe is in the university library while Ryan is at home gaming.
Chloe Smith

my skin care routine has zero efficacy. i'm literally breaking out everywhere.

Chloe Smith
Ryan
Ryan

damn that's crazy. maybe just wash your face?

💡
Chloe uses 'efficacy' to describe the failure of her products in a dramatic, academic-adjacent way typical of a liberal arts student. Ryan responds with his signature oblivious and dismissive phrase 'damn that's crazy', ignoring the nuance of her complaint.

Meanings

Noun

The ability to produce a desired or intended result; effectiveness.

The clinical trials were designed to test the efficacy of the new vaccine in preventing infection.

Examples

The clinical trials were designed to test the efficacy of the new vaccine.

Etymology

Derived from the Latin efficacia, from efficax meaning productive or effective, which is a combination of ex meaning out and facere meaning to make.

Related Words

Last Updated: May 2026Report an Error