The phrase "good for what ails you" implies that something is beneficial or healing. Therefore, antonyms for this phrase could be "harmful," "detrimental," "toxic," "poisonous," or "dangerous." These words suggest that a certain remedy or treatment might worsen one's condition, rather than helping it. Another antonym could be "ineffective," referring to a treatment or medication that has no positive effect on one's illness or ailment. Overall, antonyms for "good for what ails you" describe things that may not provide the intended benefit and, in some cases, could even cause harm.