efficacious
/ˌɛf.əˈkeɪ.ʃəs/ adjective C2- Effective; possessing efficacy. · This medicine is efficacious.
From the library
“I fancy that he had taken steps to make that draught efficacious, and that he quite relied upon your being unconscious.
He has but one fault, he is somewhat wilful; but really, on referring for the moment to what he said, do you truly believe that Mithridates used these precautions, and that these precautions were efficacious?”
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