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Definition of efficacious : Possessing the quality of being effective; productive of, or powerful to produce, the effect intended; as, an efficacious law.

Synonyms:

in force, good, effectual, sound, impelling, hard-hitting, trenchant, impressive, in effect, legal, rough-and-ready, thrive, efficient, effective, telling

productive, study at effective

Usage examples:

• That no doubt would be straight,- and efficacious. - "The Duke's Children", Anthony Trollope.
• One of these, which was supposed to be particularly efficacious, was known as " Dragon's Blood." - "Weather and Folk Lore of Peterborough and District", Charles Dack.
• For he had known that, whatever Eliot Leithgow's method of forcing him to perform the operations might be, and no matter how efficacious, the coordinated brains simply could not be put in the heads of his four assistants- because the assistants were themselves needed for the operations! - "The Passing of Ku Sui", Anthony Gilmore.