efficacious

efficacious \ef-ih-KAY-shuhs\, adjective:

Capable of having the desired result or effect; effective as a means, measure, remedy, etc.

Lawyers make claims not because they believe them to be true, but because they believe them to be legally efficacious.
-- Paul F. Campos, Jurismania

Henri IV wrote to his son's nurse, Madame de Montglat, in 1607 insisting 'it is my wish and my command that he be whipped every time he is stubborn or misbehaves, knowing full well from personal experience that nothing in the world is as efficacious'.
-- Katharine MacDonogh, Reigning Cats and Dogs: A History of Pets at Court Since the Renaissance

Plagued by rats, the citizens of Hamelin desperately seek some efficacious method of pest control.
-- Francine Prose, review of The Pied Piper of Hamelin, as retold by Robert Holden, New York Times, August 16, 1998

Efficacious is from Latin efficax, -acis, from efficere, "to effect, to bring about," from ex-, "out" + facere, "to do or make."