noun
efficacy
EHF-ih-kuh-see
noun
1
The power to produce a desired result, especially under controlled or ideal conditions.
"Clinical trials confirmed the drug's efficacy against the virus."
"The report questioned the efficacy of the new training program."
How to Use Efficacy
Learner’s notesIn plain EnglishHow well something works in practice, especially when tested scientifically or formally.
When to use it
A more formal, technical word than "effectiveness" — common in medical and scientific writing.
Easily confused with
Common pairings
clinical efficacy
proven efficacy
vaccine efficacy
Word Forms
efficacies plural
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Clinical trials confirmed the drug's _____ against the virus.
Etymology
From Late Latin efficacia, related to efficax, "capable of producing an effect."