Eating lettuce at night the next day diarrhea may be caused by acute gastroenteritis, food allergies, intestinal flora imbalance and other reasons. 1. Acute gastroenteritis: If the lettuce is not clean, the bacteria, viruses and other microorganisms in the lettuce are not removed from the body after eating it will cause the occurrence of acute gastroenteritis, followed by diarrhea, which can also be accompanied by abdominal pain, nausea, vomiting and other symptoms. 2. Food allergy: people who are allergic to lettuce will have diarrhea and other gastrointestinal symptoms after eating lettuce, which can be accompanied by acute urticaria, angioedema, swelling and itching in the oropharynx and other symptoms. 3. Intestinal flora imbalance: the balance of intestinal flora in the patient’s body is damaged, which can cause diarrhea, abdominal pain and other symptoms. Eating lettuce at night the next day diarrhea is recommended to go to the hospital, ask the doctor to make a judgment, clear the cause of the disease.