efficacious - adjective
Capable of having the desired result; effective as a method, means, or remedy.
Note: The noun form efficacy means effectiveness. For example, to gain FDA approval, a drug company must show the efficacy of the drug.
The efficacy of religion lies precisely in what is not rational, philosophic or eternal; its efficacy lies in the unforeseen, the miraculous, the extraordinary.
—Henri Frédéric Amiel Journal (1885)