Poor gastrointestinal can cause mouth ulcers, digestive system diseases is one of the causes of mouth ulcers, common diseases are gastric ulcers, ulcerative colitis, enteritis, etc. It is recommended that timely treatment of gastrointestinal diseases. 1. Bad stomach and intestines can cause gastrointestinal flora imbalance, which in turn acts on the mouth, leading to abnormal flora in the mouth, and the phenomenon of mouth ulcers may occur. 2. Poor gastrointestinal tract and indigestion can lead to a lack of vitamin B, vitamin C or trace elements, etc. Malnutrition is one of the causes of mouth ulcers. 3. Some digestive diseases, such as Crohn’s disease, duodenal ulcer, etc., in addition to causing gastrointestinal diseases, can also lead to ulcers in the mouth. 4. Bad stomach and intestines may also have acid reflux, when the reflux is severe, it may burn the oral mucosa, then ulcers may appear in the mouth.