English to English

adjective (a)

• lacking the power to produce a desired effect(adj.all)

  Example:
  Laws that are inefficacious in stopping crime.
  

  Antonym:
  efficacious
  

  source: wordnet30
• Not efficacious; not having power to produce the effect desired; inadequate; incompetent; inefficient; impotent.(adjective)

  source: webster1913