AIM:To compare the bowel cleansing efficacy of same day ingestion of 4-L sulfa-free polyethylene glycol(4-L SF-PEG)vs 2-L polyethylene glycol solution with ascorbic acid(2-L PEG+Asc)in patients undergoing afternoon colonoscopy.METHODS:206 patients(mean age 56.7 years,61%male)undergoing outpatient screening or surveillance colonoscopies were prospectively randomized to receive either 4-L SF-PEG(n=104)or 2-L PEG+Asc solution(n=102).Colonoscopies were performed by two blinded endoscopists.Bowel preparation was graded using the Ottawa scale.Each participant completed a satisfaction and side effect survey.RESULTS:There was no difference in patient demographics amongst groups.4-L SF-PEG resulted in better Ottawa scores compared to 2-L PEG+Asc,4.2 vs4.9(P=0.0186);left colon:1.33 vs 1.57 respectively(P=0.0224),right colon:1.38 vs 1.63 respectively(P=0.0097).No difference in Ottawa scores was found for the mid colon or amount of fluid.Patient satisfaction was similar for both arms but those assigned to 4-L SF-PEG reported less bloating:23.1%vs 11.5%(P=0.0235).Overall polyp detection,adenomatous polyp and advanced adenoma detection rates were similar between the two groups.CONCLUSION:Morning only 4-L SF-PEG provided superior cleansing with less bloating as compared to 2-L PEG+Asc bowel preparation for afternoon colonoscopy.Thus,future studies evaluating efficacy of morning only preparation for afternoon colonoscopy should use4-L SF-PEG as the standard comparator. AIM: To compare the bowel cleansing efficacy of same day ingestion of 4-L sulfa-free polyethylene glycol (4-L SF-PEG) vs 2-L polyethylene glycol solution with ascorbic acid (2-L PEG + Asc) in patients undergoing afternoon colonoscopy.METHODS: 206 patients (mean age 56.7 years, 61% male) undergoing outpatient screening or surveillance colonoscopies were prospectively randomized to receive either 4-L SF-PEG (n = 104) or 2-L PEG + Asc solution = 102). Colonoscopies were performed by two blinded endoscopists. Bowel preparation was graded using the Ottawa scale. Each participant completed a satisfaction and side effect survey .RESULTS: There was no difference in patient demographics amongst groups.4-L in better Ottawa scores compared to 2-L PEG + Asc, 4.2 vs 4.9 (P = 0.0186); left colon: 1.33 vs 1.57 respectively (P = 0.0224) difference in Ottawa scores was found for the mid colon or amount of fluid. Patient conditions were similar for both arms but those ass Ligation of 4-L SF-PEG with less bloating: 23.1% vs 11.5% (P = 0.0235). Overall polyp detection, adenomatous polyp and advanced adenoma detection rates were similar between the two groups. PEG provided superior cleansing with less bloating as compared to 2-L PEG + Asc bowel preparation for afternoon colonoscopy. Thus, future studies evaluating efficacy of morning only preparation for afternoon colonoscopy should use 4-L SF-PEG as the standard comparator.