The great problem of the shifting relation between passion and duty is clear to no man who is capable of apprehending it; the question whether the moment has come in which a man has fallen below the possibility of a renunciation that will carry any efficacy, and must accept the sway of a passion against which he had struggled as a trespass, is one for which we have no master-key that will fit all cases.
There are no more uses of "efficacy" in the book.
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The doctor weighed the efficacy of each treatment against potential side-effects.
They held hearings to investigate concerns about the safety and efficacy of the vaccine.