I have confidence in the remedy I propose, and only ask you to permit me to assure you of its efficacy.
Philosophers may well say, and practical men will always support the opinion, that money mitigates many trials; and if you admit the efficacy of this sovereign balm, you ought to be very easily consoled—you, the king of finance, the focus of immeasurable power.
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.