Ef"fi*ca*cy (?), n. [L. efficacia, fr. efficax. See Efficacious.]
Power to produce effects; operation or energy of an agent or force; production of the effect intended; as, the efficacy of medicine in counteracting disease; the efficacy of prayer.
"Of noxious efficacy
Syn. -- Virtue; force; energy; potency; efficiency.
© Webster 1913.