Efficacious is a formal word used to describe something—often a treatment, medicine, or remedy—that has the power to produce a desired result or effect.

// Companies like to tout the number of efficacious natural ingredients in their beauty products.

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“Baking soda is commonly used alongside detergent to fix stinky loads ... but washing soda is the typical go-to for most tough laundry jobs. Baking soda is gentler than washing soda, so it won’t be as efficacious.” — Leslie Corona, Real Simple Magazine, 29 Dec. 2023

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.