Definition

: having the power to produce a desired effect

Did You Know?

Effective, effectual, and efficient are synonyms of efficacious, but each of these words has a slightly different connotation. Efficacious suggests possession of a special quality or virtue that makes it possible to achieve a result (“a detergent that is efficacious in removing grease”). Effective stresses the power to produce or the actual production of a particular effect (“an effective rebuttal”), while effectual suggests the accomplishment of a desired result, especially as viewed after the fact (“measures taken to reduce underage drinking have proved effectual”). The last synonym, efficient, implies an acting or potential for acting that avoids loss or waste of energy (“an efficient small car”).

Examples

“And as I mentioned, at the conference today, vaccines are close by. They’re coming. I said ‘Help is on the way,’ which to me, I think, should motivate people, even more, to double down because pretty soon we’re going to get a heck of a lot of help from a very efficacious vaccine.” — Doctor Anthony Fauci, quoted on CNN, 19 Nov. 2020

“Facebook also employs fact-checking tags, but there are mixed opinions about whether these efforts are efficacious or cause users to double-down on their preconceptions.” — Scott Nover, Adweek, 28 Apr. 2020