Usually, there is no such thing as the top 10 bad things about having a colonoscopy. But doing colonoscopy may cause intestinal mucosal damage, abdominal distension, anesthesia risk and other adverse effects. 1. Intestinal mucosal damage: Because colonoscopy is an invasive examination, after entering the intestinal tract, due to the hard parts of the colonoscope at this time there may be some damage to the intestinal mucosa and produce irritation. 2. Abdominal distension: Since a large amount of gas will be introduced after colonoscopy, patients may experience abdominal distension. 3. Anesthesia risk: anesthesia is required before performing colonoscopy, and it is common that patients may experience allergic reactions such as difficulty in breathing and itchy skin after anesthesia. In addition, other adverse reactions may occur during colonoscopy, so patients are advised to go to the hospital in time to avoid delaying their condition.