efficacious

[ ef-i-key-shuhs ]

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.

efficacious

/ ˌɛfɪˈkeɪʃəs /

adjective

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

Derived Forms

• ˌeffiˈcaciousness, noun
• ˌeffiˈcaciously, adverb

Other Words From

• effi·cacious·ly adverb
• effi·cacious·ness noun
• nonef·fi·cacious adjective
• nonef·fi·cacious·ly adverb
• unef·fi·cacious adjective
• unef·fi·cacious·ly adverb

Word History and Origins

Origin of efficacious¹

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

Word History and Origins

Origin of efficacious¹

C16: from Latin efficāx powerful, efficient, from efficere to achieve; see effect

Example Sentences

It’s a huge ask of a drug that it be so safe and so efficacious that you can give it to everybody.

From Science News

By early 2020, we had a tremendous amount of data showing that in the mouse model that we had developed, these mRNA spike vaccines were really efficacious in protecting against lethal MERS-CoV infection.

From MIT Technology Review

In a release on May 25, Moderna says its vaccine is safe and efficacious among 12- to 18-year olds.

From Time

If the studies show the new vaccine could be stable and efficacious at refrigerated temperatures, that could increase the number of places that could vaccinate with Moderna’s shot.

From Time

Several days later, however, the company issued another press release including additional data that showed the vaccine is 76% efficacious in protecting people from disease.

From Time

But Michele Bachmann, who has been a wallflower of late, may point the way to a more efficacious line of attack: crony capitalism.

From The Daily Beast

I asked him if he thought cattle cars would be an efficacious means of transporting the Palestinians to their new homes.

From The Daily Beast

He inclines to the use of the tepid bath, as likely to be the most generally efficacious.

From Project Gutenberg

I know not how to offer advice, and I am incapable of giving any efficacious help.

From Project Gutenberg

I can say from experience, that it is a powerful sudorific, and very efficacious in a cold.

From Project Gutenberg

However much the theologian may believe in prayer, he cannot deny that it is less efficacious than action.

From Project Gutenberg

The commissioners regarded the land purchase act as embodying the most simple and efficacious remedy for existing evils.

From Project Gutenberg

When To Use

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.