/ ˌefɪˈkeɪʃəs; NAmE / adjective
( formal ) (of things, not of people) producing the result that was wanted or intended
SYN effective :
They hope the new drug will prove especially efficacious in the relief of pain.
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WORD ORIGIN
early 16th cent.: from Latin efficax , efficac- (from efficere accomplish, from ex- out, thoroughly + facere do, make) + -ious .