marked by qualities giving the power to produce an intended effect; "written propaganda is less efficacious than the habits and prejudices of the readers"-Aldous Huxley; "the medicine is efficacious in stopping a cough"
producing or capable of producing an intended result or having a striking effect; "an air-cooled motor was more effective than a witch's broomstick for rapid long-distance transportation"-LewisMumford; "effective teaching methods"; "effective steps toward peace"; "made an effective entrance"; "his complaint proved to be effectual in bringing action"; "an efficacious law"
Something that is efficacious is effective. The nasal spray was new on the market and highly efficacious. = effective. working in the way you intended = effective (efficax, from efficere; EFFECT)