The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

         The authority of Parliament must become inefficacious .
         . . to restrain the growth of disorders. --Burke.
   [1913 Webster]

   Note: Ineffectual, says Johnson, rather denotes an actual
         failure, and inefficacious an 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.
         [1913 Webster]

WordNet (r) 3.0 (2006):

inefficacious
    adj 1: lacking the power to produce a desired effect; "laws that
           are inefficacious in stopping crime" [ant: efficacious]