4-6 hours before colonoscopy, patients in experiment cohort began to drink the first 150ml lactulose and half hour later, drink other 150ml lactulose dissolved in 1.5 L of water at a rate of 250ml every 15 minutes.

The participants in control cohort began to drink the first 2 L of PEG at 7:00-9 PM on the day before colonoscopy at a rate of 250 mL every 15 minutes. On the day of the procedure, patients took the remaining 2 L 4-6 hours before colonoscopy.

In experiment cohort, patients do not use standard bowel preparation regimen, they totally drink 300ml lactulose.

Boston bowel preparation score (BBPS)：cleanliness of each part of the colon: 0=unprepared colon segment with mucosa not seen because of solid stool that cannot be cleared; 1=portion of mucosa of the colon segment seen, but other areas of the colon segment not well seen because of staining, residual stool, and/or opaque liquid; 2=minor amount of residual staining, small fragments of stool and/or opaque liquid, but mucosa of colon segment seen well; 3=entire mucosa of colon segment seen well with no residual staining, small fragments of stool, or opaque liquid.

We will ask patients in the form of questionnaire whether they are willing to repeat the same bowel preparation method if they need colonoscopy examination again.