Making the most of currently available bowel preparations for colonoscopy

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ABSTRACTAdequate bowel preparation is essential before colonoscopy. Choosing an agent can be confusing, since many are available. The authors review the available regimens, offer an algorithm for choosing an appropriate regimen, and provide bowel preparation instructions for patients.


  • Polyethylene glycol solutions are fast, effective, and preferred for cleansing the colon.
  • Use of split dosing, a low-volume solution, or both can increase patient acceptability without compromising efficacy.
  • Sodium phosphate can be prescribed for patients who cannot tolerate polyethylene glycol solutions, provided they are not at risk of electrolyte or fluid imbalances.
  • Enemas, bisacodyl, magnesium citrate, and metoclopramide (Reglan) can be useful as adjuncts to polyethylene glycol but by themselves are inadequate for cleansing the entire colon.
  • Educating patients about bowel preparation instructions, including correct dosing and adequate hydration, helps reduce the risk of adverse events and serious adverse events.



Figure 1. Adequate bowel preparation is essential before colonoscopy. The preparation is excellent in the top two images, allowing optimal visualization of a polyp in the top right image (arrow). In contrast, the bottom two images show inadequate bowel preparation, with semisolid or solid debris that obscures the complete view of the mucosa in spite of extensive flushing and suction.

During colonoscopy, the physician needs to inspect the entire mucosal surface. This can be done only if the bowel has been adequately prepared—ie, cleaned out (Figure 1). Inadequate bowel preparation reduces the quality of colonoscopy, raises the procedural risks, and increases the chance that polyps will go undetected.1–3 Furthermore, poor bowel preparation substantially increases costs by prolonging the procedure time and increasing the chance of an aborted examination, necessitating another procedure at an interval sooner than called for in the standard guidelines.3,4

Adequate bowel preparation depends on the right choice of bowel-cleansing agent. But with a myriad of products available, the right choice can be confusing to make.

This review discusses the currently recommended methods for bowel preparation before colonoscopy and suggests ways to solve common problems.


Colorectal cancer is the third most common cancer in the United States and the second most common cause of cancer deaths. It largely can be prevented by detecting and removing adenomatous polyps, and survival rates are significantly better when it is diagnosed while still localized.5 Early detection, through widely applied screening programs that include colonoscopy, is thought to be playing a key role in the recent decline of colorectal cancer rates in developed countries.6


Bowel preparation agents, for the most part, can be classified into one of three categories:

  • Polyethylene glycol solutions, which work as high-volume gut lavage solutions
  • Osmotic agents, such as sodium phosphate, magnesium citrate, lactulose, and mannitol, which draw extracellular fluid across the bowel wall and into the lumen
  • Stimulants (castor oil, senna, sodium picosulfte, and bisacodyl), which work by increasing smooth muscle activity within the wall of the colon.


Bowel preparation in the past consisted of dietary restriction, stimulant laxatives, and enemas. 7,8 However, these were time-consuming (taking 48–72 hours), harsh, and not very effective for adequate visualization during colonoscopy.

In 1980, Davis et al9 developed an osmotically balanced, high-molecular weight, nonabsorbable polymer given in a dilute electrolyte solution. The osmotic effect of the polymer keeps the electrolyte solution in the colon. Since little fluid is exchanged across the colonic membrane, the potential for systemic electrolyte disturbance is limited.

Since then, these solutions have become some of the preferred bowel cleansing agents worldwide.7,8 They work as an oral lavage and hence need to be taken in high volume (typically 4 L) for bowel cleansing.

Advantages and disadvantages of polyethylene glycol solutions

Polyethylene glycol solutions are more effective and better tolerated than regimens of diet combined with cathartic agents, or high-volume balanced electrolyte solutions, or mannitol-based solutions.7 Since they are osmotically balanced and do not induce substantial shifts in fluid and electrolytes, they are safe for patients who have electrolyte imbalances, advanced liver disease, poorly compensated congestive heart failure, or renal failure.

These preparations are, however, contraindicated in patients who have allergies to polyethylene glycol compounds, gastric outlet obstruction, high-grade small-bowel obstruction, significant colonic obstruction, perforation, diverticulitis, or hemodynamic instability. In addition, they are classified by the US Food and Drug Administration (FDA) as pregnancy category C and have been associated (albeit rarely) with Mallory-Weiss tear, toxic colitis, pulmonary aspiration, hypothermia, cardiac arrhythmias, pancreatitis, and inappropriate antidiuretic hormone secretion.10,11

The main disadvantages of these solutions are the large volume of fluid (4 L) that patients must drink and their unpalatable taste, which is due to sodium sulfate. The large volume of ingestion is the main reason for nausea, bloating, cramping, and vomiting with these products, which affect patient compliance and the ultimate success of colonoscopy.

Commercially available polyethylene glycol solutions

Many polyethylene glycol preparations are available today. They can be divided into those that are full-volume solutions (typically 4 L, flavored or unflavored, with sulfate or sulfate-free) and low-volume solutions (typically 2 L) (Table 1).

Standard full-volume solutions (Colyte, GoLYTELY) have been widely studied and have the most evidence of safety and effectiveness. They are also inexpensive, and most insurance companies pay for them. However, about 5% to 15% of patients do not complete the preparation, because of poor palatability, large volume, or both.7

Sulfate-free and flavored solutions. To make polyethylene glycol solutions more tolerable, sulfate-free solutions have been developed. These are less salty, more palatable, and comparable to standard solutions in terms of effective colonic cleansing.12 Sulfate-free polyethylene glycol solutions commercially available in the United States are NuLytely (flavors: cherry, lemon-lime, orange, pineapple) and TriLyte (flavors: cherry, citrus-berry, lemon-lime, orange, pineapple).

Low-volume solutions have been developed in an attempt to increase acceptability and reduce volume-related adverse effects such as bloating. For example, HalfLytely (flavor: lemon-lime) consists of 2 L of polyethylene glycol solution packaged with two bisacodyl tablets. Stimulant laxatives such as bisacodyl and magnesium citrate effectively debulk the colon of solid stool and allow a lower volume of solution to be used.13,14

Also commercially available is a preparation that contains ascorbic acid (MoviPrep). Ascorbic acid acts as a flavoring and as a cathartic, also permitting a lower volume of fluid to be used.

Studies that compared full-volume and low-volume regimens (the latter including ascorbic acid, magnesium citrate, or bisacodyl) found the low-volume regimens to be as effective and more tolerable.14–18

Combining over-the-counter polyethylene glycol 3350 laxative powder (MiraLAX) and Gatorade or Crystal Light (or another clear liquid of choice) has also been shown to improve the taste and tolerability of the preparation. Although beneficial and commonly used in certain regions of the United States, this combination is not approved for bowel preparation and its use is considered off-label.


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