Bowel Preparations: A Review for Community Pharmacists

Diana Chang, PharmD; Kevin Van, PharmD; Janette D. Lie, PharmD, BCACP; Jason P. Smith, PharmD; Kristie N. Tu, PharmD, BCPS, CGP


US Pharmacist. 2013;38(12):30-34. 

In This Article

Traditional Bowel Preparations

Bowel preparations used for colonoscopy generally contain a hyperosmotic, isosmotic, or stimulant, or a combination of these agents. Factors that should be considered when a regimen is being selected include coexisting conditions, concomitant medications, age, and patient preference. The most commonly dispensed bowel preparations are polyethylene glycol (PEG) and sodium phosphate (NaP). Poor tolerability of these traditional agents has triggered the development of newer agents, such as SUPREP, Suclear, and Prepopik.

PEG-Based Preparations

PEG-based preparations are isosmotic, nonabsorbable electrolyte solutions that cleanse the bowel through cathartic effects resulting from ingestion of a large fluid volume. Several PEG-based preparations are available, each differing in aspects such as dose volume, electrolyte content, and flavor (e.g., GoLYTELY, Colyte, PEG-3350, NuLYTELY, Trilyte). Decreased tolerance of, and compliance with, older PEG-based solutions is commonly attributed to unpleasant taste. Newer sulfate-free, PEG-based products (e.g., NuLYTELY, Trilyte) have been developed to improve palatability.[6] In addition, standard PEG-based preparations are now available with flavor enhancers to improve tolerability.[7]

Tolerability and compliance are also influenced by the large volume of fluid required for ingestion. For conventional dosing, traditional PEG-based preparations should be reconstituted with water to 4 L and consumed as 8 oz every 10 minutes until finished or rectal fluids are clear.[8] As an alternative, split dosing has successfully addressed volume-related challenges while improving efficacy (Table 1).[9–13] Split-dose regimens generally recommend the ingestion of 2 L the evening before colonoscopy, followed by 2 L the next morning. The second dose, taken approximately 5 hours before the procedure, minimizes the buildup of thick intestinal secretions in the colon, resulting in improved polyp detection.[6,12,14]

Low-volume 2-L PEG preparations such as HalfLytely (which includes bisacodyl) and MoviPrep (which contains ascorbic acid) are also alternatives to full-volume regimens. The additional ingredients act synergistically, with PEG causing a reduction in volume without compromising safety and efficacy.[10] With HalfLytely, after the bisacodyl tablets are taken, the patient should wait for a bowel movement to occur, or at least 6 hours, before consuming the PEG solution.[15] MoviPrep may be given as either a one-dose regimen the evening before colonoscopy or a split-dose regimen of 1 L followed by 0.5 L of clear liquid the evening before, then repeated the following morning.[16]

Most PEG-based solutions contain electrolytes, thereby preventing shifts in fluid and electrolytes. Some providers recommend MiraLAX—PEG-3350 containing no electrolytes—as a more tolerable alternative to the full-volume regimens. Although widely used in practice, MiraLAX as a bowel preparation is an off-label use. As a bowel preparation, MiraLAX should be mixed with 2 L of clear liquid or a sports drink and consumed as 8 oz every 10 minutes the evening before the procedure.[17] In theory, the lack of electrolyte supplementation increases the risk of electrolyte imbalances. Nevertheless, MiraLAX has been administered as one-dose and split-dose regimens. The split-dose regimen was found to be less efficacious for bowel cleansing compared with split-dose PEG.[18,19]

Overall, the efficacy and tolerability of PEG-based preparations are generally considered inferior to those of NaP preparations. Conversely, compared with NaP, PEG-based products are thought to be a safer alternative for patients at risk for developing complications secondary to fluid and electrolyte imbalances (e.g., patients with renal failure, liver failure, or congestive heart failure [CHF]). In addition, PEG-based solutions do not appear to alter the cells of the colonic mucosa, making it an effective option for patients with suspected inflammatory bowel disease.[20]

NaP Preparation

NaP was originally developed to avoid the large-volume ingestion associated with PEG preparations.[17] NaP works as a hyperosmotic laxative, drawing water into the lumen of the colon to stimulate peristalsis and catharsis. Until 2008, when the FDA issued a black box warning, Fleet Phospho-soda was a commonly used OTC NaP preparation for bowel cleansing. Because of the increased risk of acute phosphate nephropathy (APN), which may result in permanent renal impairment, this solution is no longer available on the U.S. market, according to the manufacturer (oral communication, August 2013).[8,21] Currently, only tablet formulations of NaP (Visicol, OsmoPrep) are available in the U.S. The main difference between Visicol and OsmoPrep is the presence or absence of microcrystalline cellulose (MCC).[8,22,23] This insoluble, inactive compound is thought to produce a residue that theoretically may reduce visualization of the colonic mucosa during colonoscopy.[24] Visicol contains MCC and requires the ingestion of 40 tablets to complete the regimen; OsmoPrep, which is MCC-free, requires 32 tablets (Table 1).

A common misconception about NaP products is that hydration is not necessary. As with the solution, the tablets induce cathartic effects and must be taken with 4 L of clear liquid to maintain adequate hydration and prevent serious adverse effects (AEs), such as fluid and electrolyte imbalances.[22,23,25] Additionally, these tablet formulations carry the risk of APN and should be monitored accordingly.

In general, NaP products are considered more effective and tolerable than full-volume PEG products, but they may be less safe, particularly in patients with kidney disease, CHF, advanced liver disease, or a sodium-restricted diet.[20]

Magnesium Citrate

Although not FDA-approved as a bowel preparation, magnesium citrate is a hyperosmotic laxative that has long been a component of bowel-cleansing regimens. In addition to drawing fluid into the colon and increasing motility, magnesium citrate also stimulates the release of cholecystokinin, leading to intestinal accumulation of fluid and electrolytes and eventual evacuation of the bowels.[20,24,26]

Magnesium citrate typically is not used as monotherapy for bowel cleansing. It is frequently administered with bisacodyl and has been used as an adjunct to low-volume PEG for colonoscopy preparation. Patients with abdominal pain or hemorrhage or renal dysfunction should avoid its use. Although they are considered less tolerable than NaP products, regimens containing magnesium citrate are generally well tolerated. For this reason, along with its relative affordability, magnesium citrate may be considered an alternative for appropriate patient populations.[26]