Blood in the stool once drinking may be caused by alcohol allergy, perianal disease, digestive system disease and other reasons. 1. Alcohol allergy: if the patient is allergic to alcohol, it may cause damage to the intestinal mucosa, which may lead to the symptom of blood in stool. 2. Perianal diseases: If you suffer from perianal diseases, such as hemorrhoids, anal fissure, anal fistula, perianal abscess, etc., drinking alcohol will stimulate the expansion of the perianal mucosa, which will easily cause bleeding in the stool. 3. Digestive system diseases: such as chronic colitis, proctitis, ulcerative colitis, colorectal polyps and tumors, after drinking alcohol to stimulate the mucous membrane of the gastrointestinal tract and cause gastrointestinal mucous membrane edema, congestion, vesiculation, resulting in acute rupture of blood vessels bleeding, resulting in bleeding in the stool. Patients who have blood in their stools when they drink alcohol should go to a regular hospital in time, undergo relevant examinations, clarify the cause of the disease, and take targeted treatment under the guidance of a professional doctor.