Inefficacious

1. Not efficacious; not having power to produce the effect desired; inadequate; incompetent; inefficient; impotent.
   

Ineffectual, says Johnson, rather denotes an actual failure, and inefficacious an habitual impotence to any effect. But the distinction is not always observed, nor can it be; for we can not always know whether means are inefficacious till experiment has proved them ineffectual. Inefficacious is therefore sometimes synonymous with ineffectual.