EFFICACIOUS
effective, effectual, efficacious
(adjective) producing or capable of producing an intended result or having a striking effect; “an air-cooled motor was more effective than a witch’s broomstick for rapid long-distance transportation”-LewisMumford; “effective teaching methods”; “effective steps toward peace”; “made an effective entrance”; “his complaint proved to be effectual in bringing action”; “an efficacious law”
efficacious
(adjective) marked by qualities giving the power to produce an intended effect; “written propaganda is less efficacious than the habits and prejudices...of the readers”-Aldous Huxley; “the medicine is efficacious in stopping a cough”
Source: WordNet® 3.1
Etymology
Adjective
efficacious (comparative more efficacious, superlative most efficacious)
(formal) Effective; possessing efficacy. [from 1520s]
Synonym: effective
Antonym: inefficacious
Source: Wiktionary
Ef`fi*ca"cious, a. Etym: [L. eficax, -acis, fr. efficere. See Effect,
n.]
Definition: Possessing the quality of being effective; productive of, or
powerful to produce, the effect intended; as, an efficacious law.
Syn.
– See Effectual.
– Ef`fi*ca"cious*ly, adv.
– Ef`fi*ca"cious*ness, n.
Source: Webster’s Unabridged Dictionary 1913 Edition