The purpose of the study is to assess whether a low volume polyethylene glycol (PEG) plus ascorbic acid based bowel preparation for colonoscopy is not inferior to a large volume polyethylene glycol preparation in subjects with past history of poor bowel preparation

Two key quality indicators for colonoscopy are the cecal intubation rate and the percentage of neoplastic lesions detected. Both factors are associated with adequate bowel cleansing. Poor cleansing ranged from 5% to 30% across studies, negatively affecting the efficiency of colonoscopy.

The most important factor associated with poor colonic preparation is the past history of poor bowel preparation. However, there are no recommendations on the proper type of preparation in those patients. In two non-randomized studies inadequate cleansing in the second colonoscopy ranged from 9.8% to 23%. Randomized studies comparing high volume (3-4 liters) with low volume (2 liters) PEG preparations, which are better tolerated by patients, are therefore needed before making any recommendations in this regard.

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.