Definition of efficacious adjective from the Oxford Advanced Learner's Dictionary

 

efficacious

 adjective
adjective
BrE BrE//ˌefɪˈkeɪʃəs//
 
; NAmE NAmE//ˌefɪˈkeɪʃəs//
 
(formal)
 
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(of things, not of people) producing the result that was wanted or intended synonym effective They hope the new drug will prove especially efficacious in the relief of pain. Word Origin early 16th cent.: from Latin efficax, efficac- (from efficere ‘accomplish’, from ex- ‘out, thoroughly’ + facere ‘do, make’) + -ious.
See the Oxford Advanced American Dictionary entry: efficacious