Diarrhea may cause anal fissures. An anal fissure refers to a full-thickness vertical tear in the skin of the anal canal below the dentate line, forming an ischemic ulcer. It is commonly found in young and middle-aged adults, and the exact mechanism of its development is not very clear. It is mainly related to local muscle spasms and infection following injury. The main symptoms include severe, sharp pain during and after bowel movements, along with occasional light, bright red bleeding. There may also be constipation and anal discharge. If the skin of the anal canal is chronically irritated by prolonged diarrhea, causing skin damage, it is very likely to lead to an anal fissure.