Colonoscopies are usually not done on the same day and need to be done after going through the process of booking an appointment as well as cleansing the bowel in advance. A bowel cleansing is usually required to remove all fecal matter from the body before the colonoscopy is performed. This is usually done under the supervision of a doctor using a laxative such as polyethylene glycol electrolytes or magnesium sulfate or mannitol to clean out the bowel and remove some of the fecal material from the bowel before the test. It takes a longer time to prepare and is usually less likely to be done on the same day. Therefore, it is recommended to make an appointment at the hospital in time when you are not feeling well, and make full preparations under the guidance of the doctor according to your actual situation, so as to maximize the accuracy of the examination.