Adequate quality of bowel preparation(BP) is essential for colonoscopy. In recently, Kang suggested that for low-risk patients, single dose of 2L PEG is an effective regimen for bowel preparation.However, due to the poor palatability, there still more than 30% patients with 2L regimen experienced nausea or vomiting in our center.

Oral lactulose is a treatment for constipation. Several studies have compared the effectiveness between use PEG and lactulose for colonoscopy preparation in average-risk patients. However, the data in low-risk patients is vacant. The objective of current study was to compared the effectiveness of bowel preparation and patient tolerance using lactulose and 2L PEG regimen in low-risk patients.

Inclusion Criteria:

• Patients over 18 years old and with low-risk were eligible. Patients were considered as low risk if they did not meet any of the following risk factors: age>70 years old, body mass index (BMI)>25Kg/m2, constipation, Parkinson's disease, diabetes, history of stroke or spinal cord injury, use of tricyclic antidepressant or narcotics.

Exclusion Criteria:

• (1) history of colorectal resection; (2) Known or suspected colonic stricture or obstructing tumor; (3) Known or suspected colonic perforation; (4) toxic colitis or megacolon; (5) use of prokinetic agents or purgatives within 7 days; (6) hemodynamic instability; (7) pregnancy or lactation; (8) inability to provide informed consent.