Source : Webster's Revised Unabridged Dictionary (1913)

Efficacious \Ef`fi*ca"cious\, a. [L. eficax, -acis, fr.
   efficere. See {Effect}, n.]
   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 : WordNet®

efficacious
     adj 1: 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" [ant: {inefficacious}]
     2: 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" [syn:
        {effective}, {effectual}] [ant: {ineffective}]