The Collaborative International Dictionary of English v.0.48:

Efficacious \Ef`fi*ca"cious\, a. [L. eficax, -acis, fr.
   efficere. See Effect, n.]
   Possessing the quality of being effective; productive of, or
   powerful to produce, the effect intended; as, an efficacious
   law.

   Syn: See Effectual. -- Ef`fi*ca"cious*ly, adv.
        [1913 Webster]

WordNet (r) 3.0 (2006):

efficaciously
    adv 1: in an effective manner; "these are real problems that can
           be dealt with most effectively by rational discussion"
           [syn: efficaciously, effectively] [ant:
           ineffectively, inefficaciously]