To compare 4 commonly used bowel preparations in terms of efficacy, patient tolerability and safety. All these 4 bowel preparations are likely to be efficacious and safe. However, those with lower volume are likely to be better tolerated and completed by patients.

Adequate bowel preparation is imperative for diagnostic accuracy and therapeutic safety. Several bowel cleansing preparations have been approved by Health Canada and are commonly used prior to colonoscopy. Polyethylene glycol (PEG) has been considered to be the "gold standard" for bowel preparation. It is a nonabsorbable solution which passes through the bowel without net absorption or secretion. Significant fluid and electrolyte shifts are therefore avoided. However, the large volume (4 liters), the salty taste and the sulphur smell frequently lead to poor tolerability and compliance, sometimes resulting in inadequate bowel preparation. Several other bowel cleansing preparations have been developed and include sodium phosphate (NaP), magnesium citrate, and sodium picosulphate plus magnesium oxide (PSMC)-containing preparations. NaP is a low volume hyperosmotic solution which draws plasma water into the bowel lumen to promote colonic cleansing. As a result, significant fluid and electrolyte shifts can occur. The advantage of the low volume is better patient tolerance compared to PEG. However, even in healthy individuals severe electrolyte disturbances have been reported when the 2 required doses are taken 12 hours apart. Therefore Health Canada has recommended that 2 doses of NaP be taken 24 hours apart. Patients with renal failure, congestive heart failure and liver failure should avoid this preparation (Grade I A). PSMC is a hyperosmotic saline laxative which increases intraluminal volume resulting in increased intestinal motility. Magnesium, a constituent of PSMC, stimulates the release of cholecystokinin which also stimulates intestinal motility. Since magnesium is eliminated by the kidney, patients with renal insufficiency or failure should also avoid this preparation.

Many randomized controlled trials have compared the efficacy and tolerability of various bowel cleansing regimens. Studies comparing full-volume (4 liters) PEG with low-volume (2 liters) PEG combined with magnesium citrate or bisacodyl have demonstrated equal efficacy of colonic cleansing but with improved patient tolerance. However, there is little data on how low volume PEG compares with NaP. The only meta-analysis of twenty nine trials on optimal bowel preparation concluded that NaP was more effective in bowel cleansing than 4-liter PEG or PSMC. However, there were only three trials comparing NaP to PSMC that met inclusion and exclusion criteria in this meta-analysis and the results have been conflicting. Also there were only three trials comparing PEG to PSMC in this meta-analysis with inconclusive data. Moreover, the mean number of patients per trial was small at 223, with the highest number of patients in a trial at 500.

A consensus document prepared by three leading American gastrointestinal societies (American Society of Gastrointestinal Endoscopy, American Society of Colon and Rectal Surgeons and Society of American Gastrointestinal and Endoscopic Surgeons) as well as a position paper by the Canadian Association of Gastroenterology identified the lack of an ideal bowel preparation which meets all these criteria. Furthermore, the need for further studies was identified in the following areas:

1. Two-liter PEG vs NaP
2. PSMC vs NaP

Outpatients who need to undergo routine colonoscopy will be recruited. Each patient will be randomly assigned to one of the four bowel preparations after they have given consent to participate in the study:

1. Group 1. 4L PEG.
2. Group 2. 2L PEG plus 4 tablets of bisacodyl.
3. Group 3. 90 cc NaP.
4. Group 4. PSMC plus 1 bottle of Mg-citrate (300 cc).

Inclusion Criteria:

• Male or female between the ages of 18 and 75.
• Planned elective outpatient colonoscopy.

Exclusion Criteria:

• History of renal insufficiency or abnormal creatinine clearance with GFR <59.
• History of congestive heart failure.
• History of acute coronary syndrome or unstable angina.
• History of liver cirrhosis or ascites.
• Chronic lasix therapy.
• History of colorectal resection.
• Known or suspected bowel obstruction, megacolon or ileus