The Collaborative International Dictionary of English v.0.48:

Inefficacy \In*ef"fi*ca*cy\, n. [L. inefficacia. See In- not,
   and Efficacy.]
   Lack of power to produce the desired or proper effect;
   inefficiency; ineffectualness; futility; uselessness;
   fruitlessness; as, the inefficacy of medicines or means.
   [1913 Webster]

         The seeming inefficacy of censures.      --Bp. Hall.
   [1913 Webster]

         The inefficacy was soon proved, like that of many
         similar medicines.                       --James
                                                  Gregory.
   [1913 Webster]

WordNet (r) 3.0 (2006):

inefficacy
    n 1: a lack of efficacy [syn: inefficacy, inefficaciousness]
         [ant: efficaciousness, efficacy]