Adequate bowel preparation is critical for successful colonoscopy and a large volume of PEG was required for bowel preparation in patients undergoing colonoscopy. The investigators conducted a a prospective, randomized, controlled study to compare low dose of oral magnesium sulfate solution with high dose PEG. The investigators found that patients who took low dose of oral magnesium sulfate solution had similar bowel preparation quality to patients who took PEG,but accompanied with fewer adverse events and better tolerance.

Inclusion Criteria:

• patients who underwent colonoscopy

Exclusion Criteria:

• known or suspected bowel stricture or obstruction;
• history of colorectal surgery;
• significant gastroparesis or gastric outlet obstruction;
• toxic colitis or megacolon;
• severe acute inflammatory bowel disease;
• active gastrointestinal bleeding;
• suspected or confirmed chronic renal insufficiency and heart illness;
• disturbance of electrolytes ;
• unable to provide informed consent