adjective (a)

• marked by qualities giving the power to produce an intended effect(adj.all)

  Example:
  Written propaganda is less efficacious than the habits and prejudices...of the readers.
  The medicine is efficacious in stopping a cough.
  

  Antonym:
  inefficacious
  

  source: wordnet30
• producing or capable of producing an intended result or having a striking effect(adj.all)

  Example:
  An efficacious law.
  

  Synonym:
  effective, effectual
  Antonym:
  ineffective, ineffectual, uneffective
  

  source: wordnet30
• Possessing the quality of being effective; productive of, or powerful to produce, the effect intended; as, an efficacious law.(adjective)

  source: webster1913