A colonoscopy bowel prep is a cleaning substance that completely empties the contents of the colon by causing frequent, high-volume diarrhea, states Harvard Health Publications. While the exact instructions vary by doctor preference and the time of the procedure, the prep usually starts the afternoon or evening prior.
Examples of bowel-cleaning preparations include Golytley, Nulytely, Miralax, Visicol and magnesium citrate, according to Harvard Health Publications. To ensure a successful bowel prep, it's important for patients to eat a low-fiber diet during the few days prior to the procedure. Patients must also allow adequate time and privacy and plan on remaining close to a restroom while the bowel prep is being completed.