Diarrhea is a common post-colonoscopy side effect that may last from a few hours to a couple of days, according to Healthtap. It is caused by the laxatives or water used to prep the colon prior to the procedure.
A colonoscopy allows a gastroenterologist to examine the inside of the colon for signs and symptoms to confirm diagnosis and treat disorders, as explained by MedicineNet.com. For accurate results, the colon needs to be completely cleaned before a colonoscopy. This entails drinking a large volume of clear liquid, taking a laxative or getting an enema, which all cause loose, watery stool.