Drinking mannitol directly without following medical advice can be harmful to the body. Mannitol refers to mannitol injection, which is usually administered intravenously and is mainly used for dehydration, lowering intraocular pressure, lowering intracranial pressure, and osmotic diuretic effect. Mannitol is taken orally and can be used for preoperative bowel preparation. Adverse reactions to mannitol injection are commonly associated with water and electrolyte disturbances, as well as chills, fever, dysuria, thrombophlebitis, tissue edema, and skin necrosis. It is contraindicated in anuric patients who have been diagnosed with acute tubular necrosis, in those with severe water loss, in those with active intracranial hemorrhage, in those with acute pulmonary edema, or in those with severe pulmonary stasis. In case of adverse reactions caused by oral or injectable mannitol, it is recommended to cooperate with a medical professional for medical intervention as soon as possible to avoid serious adverse consequences.