Adequate quality of bowel preparation(BP) is important for colonoscopy. Several guidelines recommend that split-dose of 4L PEG should be used as a standard regime for BP. However, the high-volume PEG still results in lower compliance to the regime and increased cost.

Some high risk factors for inadequate BP have been identified, including old age, constipation, diabetes, the use of narcotics and prior history of inadequate BP. For average-risk patients without the high risk factors, the procedure of BP could be easier. In the previous study, with the use of single dose of 2L PEG, more than 90% of average-risk patients achieved adequate BP. Here investigators hypothesized that compared with the standard split dose of 4L PEG, single dose of 2L PEG may be not inferior in BP quality while may be accompanied with better tolerability.

Inclusion Criteria:

• Patients undergoing colonoscopy;

• Patients with average risks for inadequate BP (Patients were identified average-risk if they did not meet any factor of the following risk factors):

  • Constipation
  • Diabetes
  • Parkinson's disease
  • History of stroke or spine cord injure
  • Prior history of inadequate bowel preparation
  • BMI>25
  • Use of tricyclic antidepressant or narcotics

Exclusion Criteria:

• History of colorectal resection;
• Suspected colonic stricture or perforation;
• Incomplete or complete bowel obstruction;
• Use of prokinetic agents or purgatives within 7 days;
• Toxic colitis or megacolon;
• Pregnancy or lactation;
• Unable to give informed consent;
• Haemodynamically unstable.