efficacious

effective, effectual, efficient, efficacious mean producing or capable of producing a result. effective stresses the actual production of or the power to produce an effect <an effective rebuttal>. effectual suggests the accomplishment of a desired result especially as viewed after the fact <the measures to stop the pilfering proved effectual>. efficient suggests an acting or a potential for action or use in such a way as to avoid loss or waste of energy in effecting, producing, or functioning <an efficient small car>. efficacious suggests possession of a special quality or virtue that gives effective power <a detergent that is efficacious in removing grease>.