Source : Webster's Revised Unabridged Dictionary (1913)

Inefficacious \In*ef`fi*ca"cious\, a. [Pref. in- not +
   efficacious: cf. F. inefficace, L. inefficax.]
   Not efficacious; not having power to produce the effect
   desired; inadequate; incompetent; inefficient; impotent.
   --Boyle.

         The authority of Parliament must become inefficacious .
         . . to restrain the growth of disorders. --Burke.

   Note: Ineffectual, says Johnson, rather denotes an actual
         failure, and inefficacious and habitual impotence to
         any effect. But the distinction is not always observed,
         nor can it be; for we can not always know whether means
         are inefficacious till experiment has proved them
         ineffectual. Inefficacious is therefore sometimes
         synonymous with ineffectual.

Source : WordNet®

inefficacious
     adj : lacking the power to produce a desired effect; "laws that
           are inefficacious in stopping crime" [ant: {efficacious}]