In-efficacious means something that is ineffective or incapable of producing the desired result. Antonyms for in-efficacious are effective, efficient, successful, powerful, fruitful, and productive. An effective solution means a successful solution or productive one, and an unproductive solution would be described as in-efficacious. Efficient means it is done in a way that achieves maximum productivity with minimum wasted effort, while a solution that takes a lot of time and resources and does not produce the desired result is in-efficacious. It is important to know the antonyms of in-efficacious to be able to choose the right words when describing anything that is capable of achieving the expected result.