Source : Webster's Revised Unabridged Dictionary (1913)

Inefficacy \In*ef"fi*ca*cy\, n. [L. inefficacia. See {In-} not,
   and {Efficacy}.]
   Want of power to produce the desired or proper effect;
   inefficiency; ineffectualness; futility; uselessness;
   fruitlessness; as, the inefficacy of medicines or means.

         The seeming inefficacy of censures.      --Bp. Hall.

         The inefficacy was soon proved, like that of many
         similar medicines.                       --James
                                                  Gregory.

Source : WordNet®

inefficacy
     n : a lack of efficacy [syn: {inefficaciousness}] [ant: {efficacy}]