The term "in effectual" is used to describe something that is ineffective, useless or futile. Antonyms for this term would describe something that is effective, useful or successful. Words that could be used as antonyms for "in effectual" include "productive," "efficient," "potent," "powerful," "effective," "successful," "fruitful," "valuable," and "beneficial." These words are often used to describe things that have a positive impact and are able to achieve their intended purpose. It is important to use antonyms like these when one wants to convey the opposite of "in effectual" in a clear and concise way.