efficacious

American  

[ef-i-key-shuhs] / ˌɛf ɪˈkeɪ ʃəs /

adjective

1. capable of having the desired result or effect; effective as a means, measure, remedy, etc..

   The medicine is efficacious in stopping a cough.

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efficacious British  

/ ˌɛfɪˈkeɪʃəs /

adjective

1. capable of or successful in producing an intended result; effective as a means, remedy, etc

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Usage

What are other ways to say efficacious? The adjective efficacious describes things that are capable of having the desired result or effect. But efficacious isn’t quite the same as effectual, effective, or efficient! Find out why on Thesaurus.com. 

Other Word Forms

• efficaciously adverb
• efficaciousness noun
• nonefficacious adjective
• nonefficaciously adverb
• unefficacious adjective
• unefficaciously adverb

Etymology

Origin of efficacious

First recorded in 1520–30; from Latin efficāci- (stem of efficax ) “effectual.” See efficient, -acious

Example Sentences

Lilly is also developing a highly efficacious injection called retatrutide that targets GIP and GLP-1 — both of which are activated by tirzepatide — in addition to glucagon.

From MarketWatch • Apr. 1, 2026

Instead of manipulating government for their own narrow interests, the rich can add a distinctively independent-minded, public-spirited and efficacious set of voices to the national debate.

From The Wall Street Journal • Feb. 17, 2026

The team calculated that the high dose of L9LS was 70% efficacious at preventing infection and 77% efficacious at stopping disease.

From Science Magazine • May 1, 2024

But there comes a point where, if that’s not efficacious, you’re going to have to do something more publicly in the courtroom.

From Salon • Feb. 2, 2024

She favored a black Brazilian priest who, she claimed, was especially efficacious.

From "Hunger of Memory" by Richard Rodriguez