This study will be unicenter, randomized and single blind. It focuses on studying the effectiveness of four preparations for bowel cleansing before colonoscopy. Polyethylenglycol (PEG) will be compared as a gold standard of bowel cleansing with low volume preparations: Sulfate-based solution (SBS), low volume PEG + ascorbic acid and solution of magnesium citric-acid and sodium picosulfate. The effectiveness will be rated according to a quality of bowel preparation (Boston Bowel Preparation Scale) and detection of a colorectal neoplasia. The aim of this project is to compare the effectiveness of each four preparations.