This is an investigator-initiated, single site (MCG only), single-blinded prospective study comparing the efficacy of Lubiprostone and 4L PEG (Nulytely) to 4 Liters (4L) PEG alone on preparation quality in patients with known diabetes undergoing colonoscopy. Diabetic outpatients who require a colonoscopy will be randomized to one of the two groups of 60 (total of 120 patients):