Effective, effectual, and efficient are synonyms of efficacious, but each of these words has a slightly different connotation. Efficacious suggests possession of a special quality or virtue that makes it possible to achieve a result ("a detergent that is efficacious in removing grease"). Effective stresses the power to produce or the actual production of a particular effect ("an effective rebuttal"), while effectual suggests the accomplishment of a desired result, especially as viewed after the fact ("measures taken to reduce underage drinking have proved effectual"). The last synonym, efficient, implies an acting or potential for acting that avoids loss or waste of energy ("an efficient small car").