efficacious

adjective

ef·​fi·​ca·​cious ˌe-fə-ˈkā-shəs 

: having the power to produce a desired effect

an efficacious remedy

efficaciously adverb

efficaciousness noun

Did you know?

If you guesstimate that efficacious is the effect of combining effective with the suffix -ious, you’re on the right track. Efficacious came to English from the Middle French word efficace (or that word’s Latin source, efficāc- or efficāx), meaning “effective.” (These words ultimately trace back to the Latin verb efficere, “to make, bring about, produce, carry out.”) English speakers added -ious to effectively create the word we know today. Efficacious is one of many, er, eff words that mean “producing or capable of producing a result.” Among its synonyms are the familiar adjectives effective and efficient. Efficacious is more formal than either of these; it’s often encountered in medical writing where it describes treatments, therapies, and drugs that produce their desired and intended effects in patients.

Synonyms

• effective
• effectual
• efficient
• fruitful
• operative
• potent
• productive

Choose the Right Synonym for efficacious

effective, effectual, efficient, efficacious mean producing or capable of producing a result.

effective stresses the actual production of or the power to produce an effect.

an effective rebuttal

effectual suggests the accomplishment of a desired result especially as viewed after the fact.

the measures to stop the pilfering proved effectual

efficient suggests an acting or a potential for action or use in such a way as to avoid loss or waste of energy in effecting, producing, or functioning.

an efficient small car

efficacious suggests possession of a special quality or virtue that gives effective power.

a detergent that is efficacious in removing grease

Examples of efficacious in a Sentence

taking a cookie break while studying is one of the most efficacious ways of rejuvenating the mind that I have ever discovered

Recent Examples on the Web

On the other hand, there are a few contemporary beauty labels making waves for their efficacious offerings. —Kiana Murden, Vogue, 17 June 2024

Body and face treatments draw on the island’s rich bounty of lavender, eucalyptus and honey, for an experience that is aromatic, efficacious, and deeply moving. —Todd Plummer, Condé Nast Traveler, 8 June 2024

These immensely costly and invasive programs came at the expense of more efficacious policies such as full vaccination programs for the elderly. —Yanzhong Huang, Foreign Affairs, 16 Feb. 2023

Should the candidate be found to be safe and efficacious in clinical testing, IAVI is committed to making its Lassa vaccine affordable and accessible to all populations in need. —Fort Worth Star-Telegram, 4 Apr. 2024

Word History

Etymology

borrowed from Middle French efficace "effective" or its source Latin efficāc-, efficāx "capable of fulfilling a function, effective" (from efficere "to make, bring about, produce, carry out" + -āc-, -āx, deverbal suffix denoting habitual or successful performance) + -ious — more at effect entry 1, audacious

First Known Use

1528, in the meaning defined above

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Kids Definition

efficacious

adjective

ef·​fi·​ca·​cious ˌef-ə-ˈkā-shəs 

: having the power to produce a desired result

an efficacious remedy

efficaciously adverb

efficaciousness noun