Diarrhea is also called diarrhea, and dry mouth is usually considered a case of dehydration. Diarrhea is often accompanied by the loss of water and electrolytes in the body, and prolonged diarrhea can lead to different degrees of dehydration symptoms, which may result in dry mouth, and generally the dehydration symptoms will gradually subside after timely hydration. Prolonged vomiting and diarrhea will lead to a large loss of digestive juices, and if sufficient water is not replenished, hypertonic dehydration will result, i.e., water loss accompanied by electrolyte loss. In hypertonic dehydration, the high osmolarity of extracellular fluid will stimulate the hypothalamic thirst center and manifest as symptoms of thirst. This is accompanied by the release of antidiuretic hormone and a significant decrease in urine content. Mild dehydration will turn into isotonic dehydration after timely rehydration, and the condition is usually not serious. However, gradual increase in unresolved dehydration may also lead to symptoms other than thirst, including dry skin and mucous membranes, facial flushing, and restlessness. Psychoneurological symptoms may also occur when dehydration involves brain cells, and in severe cases, coma may even occur. In the case of dry mouth with diarrhea, the first thing to do is to treat the original disease. Severe dehydration needs to be treated with rehydration under the guidance of a doctor according to the patient’s individual condition.