This is a single center randomized controlled trial to compare the colon cleansing quality determined by the Boston Bowel Preparation Scale achieved by two strategies in patients with high risk of poor colon cleansing defined as those patients with a score> 1.225 following a predictive score previously published: one group will receive an intensified split-dose 4 L polyethylene glycol solution (PEG) plus bisacodyl and 3 days of fiber-free diet and the other group a split-dose 2 L PEG solution with ascorbic plus bisacodyl and 1 day of fiber-free diet.

Patients with a score ≤ 1.225 will receive a split-dose 2 L PEG solution with ascorbic plus bisacodyl and 1 day of fiber-free diet.

This is a prospective, randomized, single-blind phase IV study in which all outpatients scheduled for a colonoscopy will be given a different bowel cleansing strategy (conventional or intensified) according to a scoring system already validated in the investigator's center, designed with variables independently associated with poor bowel cleansing, constipation, abdominal/pelvic surgery, comorbidity and to be on antidepressant treatment. According to this system, a score equal to or less than 1.225, predicts with a high confidence (negative predictive value = 88%) a suitable cleansing quality with a standard preparation protocol. Therefore, these subjects will be advised of a conventional preparation (low fiber diet the day before the examination and low volume preparation consisting of 2 L of PEG with ascorbic acid). Subjects with a score greater than 1.225 will be randomized to receive an intensive bowel preparation (low fiber diet three days prior to colonoscopy and large bowel preparation consisting of 4 L PEG with bisacodyl) or a conventional one (low fiber diet the day before the examination and 2 L of PEG with ascorbic acid). The choice of low-volume preparation as a control is supported by the absence of significant differences in quality of cleanliness in patients with a score > 1.225 among patients prepared with high or low volume and that low volume preparation appears to improve tolerance and compliance.

A researcher will offer to participate in the study to all outpatients with a scheduled colonoscopy, who meet all the inclusion criteria and none of the exclusion criteria. The researchers will explain the purpose of the study and will ask to sign the informed consent. They will give verbal and written information on the bowel preparation strategy. Thus, according to the allocation group, participants must comply with a large volume cleansing strategy or a conventional one. For patients with a low score (≤ 1.225) a conventional preparation will be recommended.

Patients must complete a baseline questionnaire at the inclusion visit and another questionnaire at the colonoscopy visit.

The hypothesis of the study is that in patients with a high risk of poor bowel preparation, the large bowel based preparation strategy is superior to the conventional bowel preparation in achieving an acceptable bowel cleansing assessed by a validated scale (Boston Bowel Preparation Score).

Inclusion Criteria:

• Age ≥ 18 years
• Outpatient colonoscopy
• Willing to participate (informed consent signed).

Exclusion Criteria:

• Intestinal perforation
• Poorly controlled arterial hypertension (HTAS> 180 HTAD> 100)
• Congestive heart failure
• NYHA III-IV
• Acute hepatic failure
• End-stage renal disease (creatinine <15 ml / min dialysis or pre-dialysis)
• Pregnancy or breastfeeding
• Known hypersensitivity reaction to drug components
• Phenylketonuria or Glucose-6-phosphate dehydrogenase deficiency
• Dementia with difficulty in intake of Preparation
• Past history of poor bowel preparation colonic cleanliness
• Inability to follow the instructions

Only the endoscopists in charge of rating the bowel cleansing during the colonoscopy will be masked