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

Explanation

When you really want to do something right, really nail it, really get at what you were going for — you're trying to be efficacious, or produce the effect you intended. The word efficacious is made up of the Latin efficere meaning "accomplish" and the suffix -ious meaning "full of." Something that's efficacious is full of accomplishment. An efficacious medicine is one that cures you of the ailment you had. An efficacious recipe is one that comes out the way you intended it to taste. Remember that efficacious has two fs and two cs in it and you'll be efficacious in your spelling of the word.

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

"Our findings indicate that the combination of cretostimogene grenadenorepvec and pembrolizumab offers a unique, efficacious and durable bladder-preserving alternative strategy to radical cystectomy."

From Science Daily • Jun. 6, 2024

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

One of the best and most efficacious remedies would be for the person who has taken possession of them to go there to live.

From "The Prince" by Niccolò Machiavelli