efficacious

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ef·fi·ca·cious / ˌefiˈkāshəs/ • adj. formal (typically of something inanimate or abstract) successful in producing a desired or intended result; effective: the vaccine has proved both efficacious and safe.DERIVATIVES: ef·fi·ca·cious·ly adv.ef·fi·ca·cious·ness n.

efficacious

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efficacious XVI. f. L. efficāx, -āc, f. efficere; see EFFECT, -ACIOUS.
So efficacity XV. — F. — L. efficient making a thing what it is XIV; adequately operative or skilled XVIII. —prp. of L. efficere. efficiency XVI.