Most tonic means having the greatest invigorating effect on the body or mind. The antonyms of most tonic are words that denote the opposite of its meaning. For instance, the words debilitating, enervating, exhausting, fatiguing, and weakening are antonyms of most tonic. These words indicate a state of weakness, exhaustion or lethargy instead of vigor and energy that most tonic implies. Other antonyms of most tonic include depressing, demotivating, and disheartening. These words suggest low spirits, dejection or a loss of enthusiasm. In conclusion, knowing antonyms of a word like most tonic can help broaden one's vocabulary and understanding of words.