Definition of efficacy:
expertness, skill, competency, effectuality, efficiency, power, capability, susceptibility, cogency, potency, ability, might, capacity, cleverness, effectualness, efficaciousness, faculty, influence, readiness, potency, force, productiveness, qualification, aptitude, energy, dexterity, affect, effect, talent, strength, effectiveness.
Many of the Italian artists that I knew believed in the efficacy of coral as a talisman and always kept a bit of it about them to rub " for luck" just before they went on for their part of the performance.- "Memoirs of an American Prima Donna", Clara Louise Kellogg.
When a man is in a rage and wants to hurt another in consequence, he can always regard himself as the civil arm of a spiritual power, and all the more easily because there is real need to assert the righteous efficacy of indignation.- "Impressions of Theophrastus Such", George Eliot.
Newspaper correspondents, of course, are the natural enemies of Governments in time of war; and the experience of the year 1870 shows that the fate of Empires may depend on the efficacy of the arrangements for controlling them.- "The Development of the European Nations, 1870-1914 (5th ed.)", John Holland Rose.