Being efficacious means being effective in achieving a desired result. The antonyms of efficacious include ineffective, weak, powerless, useless, and fruitless. Ineffective implies a lack of ability to produce the desired result. Weak, powerless, and useless refer to a lack of strength or capability to bring about a significant outcome. Fruitless means pointless or unproductive. It is important to note that these antonyms are not synonyms, and using them interchangeably can change the actual meaning of a sentence or statement. Therefore, it is crucial to understand the proper usage of these antonyms to ensure coherence and clarity in one's communication.