Efficacious means producing the result it is meant to produce. An efficacious treatment, highly efficacious, proved efficacious.
It belongs to medicine, where efficacy is measured in trials under ideal conditions and effectiveness is how it works in the real world. The two are not the same, and a drug can be efficacious and still disappoint.
1adjectiveefficaciousProducing the result it is meant to produce.
“Of all the remedies she tried, only rest and plenty of water proved efficacious.”
efficacious is a quality.
The Latin is to bring about, the same root as effect. Something efficacious has the effect it was meant to have.
hooks ontoeffect
Latin efficax means effective, from efficere, to accomplish, from ex plus facere, to make.
Latin
efficāx — efficacious
Old French
efficacieux
English, today
efficacious
Built from
From Old French efficacieux (whence -acious), from Latin efficāx (“efficacious”) -ous, from efficere (“to effect, to accomplish”); see effect.
Etymology from Wiktionary, CC BY-SA