INEFFICA'CIOUS, a. [L. inefficax; in and efficax, efficio, to effect; ex and facio, to make.]

Not efficacious; not having power to produce the effect desired, or the proper effect; of inadequate power or force.

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 cannot always know whether means are inefficacious, till experiment has proved them ineffectual; nor even then, for we cannot be certain that the failure of means to produce an effect is to be attributed to habitual want of power, or to accidental and temporary causes.

Inefficacious is therefore sometimes synonymous with ineffectual.