Colon Cleansing Quality of Polyethylene Glycol Compared With Polyethylene Glycol Plus Ascorbic Acid.

Inclusion Criteria: * Age\> 18 years * Outpatients with a history of past poor bowel prep, defined according to the Boston scale as a score less than 5. * Signed informed consent Exclusion Criteria: paralytic ileus, intestinal obstruction, megacolon, poorly controlled hypertension (systolic pressure\> 180, dyastolic pressure\> 100), congestive heart failure, acute liver failure, end stage renal disease (dialysis or pre-dialysis), New York Heart Association class III-IV, pregnancy, diagnosis of phenylketonuria, diagnosis of glucose-6-phosphate dehydrogenase deficiency, dementia. History of poor preparation in the previous colonoscopy, colon resection, less than 75% intake of the bowel preparation in the index colonoscopy, refusal to participate in the study, patients in whom a new colonoscopy is not indicated despite a poor bowel preparation(i.e. ileal Crohn disease with poor large bowel preparation), lack of compliance with the bowel preparation schedule.