The term "in efficacy" means ineffective or not producing the desired outcome. Some antonyms for this phrase include effective, useful, efficient, productive, successful, powerful, potent, and influential. These words indicate a positive result, suggesting that something is capable of producing the effect that is desired. Conversely, "in efficacy" refers to something that is not working as intended or producing the desired impact. These antonyms are useful for expressing the opposite meaning of "in efficacy" and can be used to describe a diverse range of situations, products, and services that either work or don't work as intended.