Patients participating in the study shall go on with their usual diet prior to colonoscopy. Three days before avoid consuming fruits, vegetables, seeds and cereals. A low residue soft and liquid diet is allowed.

The day before the study, the group receiving lactulose shall take 4 tablets of Bisacodyl (5 mg) the day before the study at 6pm. Then, at 8pm a bottle of 250 mL (162.5g lactulose) of lactulose shall be dissolved in 600 mL of water and all contents (850 mL) shall be taken in 2 hours. At 10pm they have to drink 2 liters still water.

Patients participating in the study shall go on with their usual diet prior to colonoscopy. Three days before avoid consuming fruits, vegetables, seeds and cereals. A low residue soft and liquid diet is allowed.

The day before the study, the group receiving polyethylene glycol (PEG) should take 4 tablets of Bisacodyl (5 mg) at 4pm. Then, at 6pm, 3 bottles of PEG (Polyethylene glycol 3350 60 g; sodium chloride 1.46 g; potassium chloride 745 mg; Sodium bicarbonate 1.68 g; anhydrous sodium sulfate 5.68 g; pineapple flavoring 483 mg) shall be dissolved in 3 liters of water (1 bottle per liter) and they shall take half the preparation, that is 1 liter and a half in 2 hours. At 10pm they shall take the remaining contents in 2 hours.

4 tablets of Bisacodyl (5 mg) + 250 mL (162.5g lactulose) of lactulose + 600 mL of water + 2 L of water

Compare the level of bowel preparation (by Boston Bowel Preparation Scale) between two groups with lactulose and Polyethylene Glycol.

The minimum and maximum value is 0 and 9, respectively. A proper preparation is consider a Boston Bowel Preparation Scale ≥ 2 scores in each colonic segment. Unsuitable preparation a Boston Bowel Preparation Scale < 2 scores in any of the segments.

Score 0: is considered a segment of unprepared colon with mucosa that cannot be observed given the presence of solid stool which does not disappear after cleansing.

Score 1: part of the colonic mucosa is observed; other areas cannot be suitably assessed given the presence of staining, residual matter and/or opaque liquid.

Score 2: little remaining residual matter and small matter fragments which do not prevent correct visualization of colonic mucosa in said segment.

Score 3: colonic mucosa which can be entirely observed with no residual matter nor small fragments of matter and opaque liquid.

Two researchers shall review the endoscopic study videos and they shall complete the Boston Bowel Preparation Scale inter-observer questionnaire. Results obtained by the endoscopist in charge of the study and the observers shall be compared so as to establish inter-observer variability. Researchers shall analyze one month later again all colonoscopies recorded thus establishing Boston Bowel Preparation Scale of each endoscopy. Then results previously established shall be compared so as to establish intra-observer variability.

The minimum and maximum value is 0 and 9, respectively. A proper preparation is consider a Boston Bowel Preparation Scale ≥ 2 scores in each colonic segment. Unsuitable preparation a Boston Bowel Preparation Scale < 2 scores in any of the segments.

To assess tolerance it will be considered: good tolerance those patients with 3 or less symptoms with no major criterion; regular tolerance those patients with 4-5 symptoms, without major criterion and bad tolerance those patients with 6 or more symptoms or 1 or more major criteria.