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February 2015, Vol 3, No 2 - Inside Pharmacy
Lauren Miller, PharmDc

Bowel preparations are used to empty the colon before colonoscopies.

The effectiveness of the bowel preparation determines the accuracy of the colonoscopy. Inappropriate bowel preparation leads to a third of incomplete procedures, negatively impacting colon cancer screenings.1,2 The ideal preparation should empty the colon rapidly without causing damage to the gastrointestinal mucosa, or shift fluid-electrolyte balance.1 Other features of an ideal preparation include patient-specific considerations such as product tolerability and cost.

Choosing the Right Agent

None of the preparations currently available meet all of the ideal criteria, which is why it is important for pharmacists and retail clinicians to evaluate several factors when considering agents. Physicians tend to choose preparations based on optimum patient compliance, whereas patients prefer preparations with a low fluid volume, that is palatable, and available at a lower cost.1

Comorbid conditions and medications should also be taken into consideration for each patient when choosing a bowel preparation, to ensure that it is safe to use.

Options Available

There are several prescription and over-the-counter (OTC) bowel preparations available for patient use.

Prescriptions are more commonly used and include polyethylene glycol (PEG) derivatives such as HalfLytely, Golytely, and Colyte.1 Other prescriptions include the sodium phosphate derivatives OsmoPrep and Visicol. Although these options require a prescription, they are more likely to be used than OTC products. OTC agents available include magnesium citrate and a PEG derivative, Miralax (PEG-3350). Although these options are more accessible for patients, they must be used in conjunction with a stool softener (eg, bisacodyl).1,3

There are 3 types of bowel preparations, which are differentiated based on their mechanism of action. The gold standard for colonoscopy preparations are PEG derivatives.1 PEG is an isosmotic agent, and is safe and effective to cleanse the bowels without causing major shifts in the fluid-electrolyte balance. The standard PEG preparation is a 4-L solution that patients consume in 8-oz proportions at 10-minute intervals.

Two examples of this formulation are available on the market: Golytely and Colyte.1,2 These agents cost between $25 to $50 and coverage is dependent on the patient’s insurance plan.4 In addition to keeping osmotic balance, other benefits associated with these agents include no change in the colonic mucosa, making it safe to use in patients with irritable bowel syndrome. They are also generally well tolerated by patients.1 Some disadvantages include the salty taste and large volume the patient must consume. Cautions associated with the use of these agents include concurrent use with angiotensin-converting enzyme inhibitors (ACEIs) or potassium-sparing diuretics, and elderly patients or patients with diabetes.5 Although adverse effects are often rare, nausea, vomiting, and increased plasma volume have been reported.3

Newer variations of PEG to help improve patient compliance and tolerability of preparation have been brought to the market. Sulfate-free derivatives were developed to improve the smell and salty taste of the preparations, including Nulytely and Trilyte.1 They have the same mechanism of action as PEG, as well as a comparable cleansing effect, but have increased tolerability because they are more palatable to patients. Another derivative was created to help address the patient concerns about the large volumes of PEG that have to be consumed.1 Halflytely was developed as a 2-L solution that is used in conjunction with bisacodyl tablets. The decreased volume has been shown to decrease patient concerns of cramping and bloating while providing equal efficacy in colonoscopy preparation.1

The other prescription available for colonoscopy preparation is sodium phosphate (NaP). NaP was developed as a hyperosmotic agent in both tablet and aqueous formulation.1 It works by drawing water into the bowel lumen to promote cleansing.1 The tablet formulation, Visicol, is taken as 32 to 40 tablets split between the night before and the day of the procedure.1 OsmoPrep, the liquid form, is taken as 2 doses of 30 to 45 mL, 10 to 12 hours apart.1-3 Both dosages of NaP are expensive, with OsmoPrep costing almost $600.4 Benefits of using this agent include the smaller volume and improved taste for patients, but it is associated with downfalls of significant fluid/electrolyte shifts and side effects that can mimic inflammatory bowel disease (IBD).1,2 Before recommending this agent, patients’ other comorbid conditions need to be addressed.2 Because NaP can cause phosphate nephropathy, it should be used with caution in patients with renal failure or who are receiving an ACEI/angiotensin II receptor blocker.1,2 Other conditions to use caution with include children aged <5 years, hepatic dysfunction, recent myocardial infarction/unstable angina, congestive heart failure, or IBD. NaP is contraindicated in several of these conditions because of its adverse effects, such as colonic ulcers, dehydration, and hyperkalemia.1-3 Prescribers should thoroughly evaluate patients’ underlying conditions before recommending this agent.

OTC agents available for patients include a long-term saline laxative, magnesium citrate.1 Magnesium citrate is a hyperosmotic agent similar to NaP that increases intestinal motility by increasing the volume of the intestinal lumen.1 It is typically prescribed in 1.5-L bottles the night before the procedure and 1.5-L bottles the morning of the procedure.6 As an OTC agent, magnesium citrate is more affordable and easier for patients to access, but it also comes with some precautions. Because of the hyperosmotic mechanism, significant fluid-electrolyte shifts can be seen with this agent, as well as several drug and disease state interactions. Magnesium is renally excreted and contraindicated for use in kidney, liver, and congestive heart failure. Caution should be exercised because of the side effects this agent can have (eg, cramping, irregular heart rhythm, and fluid retention).3,6

Miralax is a PEG derivative that was developed as an OTC agent and is being used more often in conjunction with bisacodyl for bowel preparation.1 Miralax is a low-volume PEG agent without added electrolytes. It has shown equal efficacy to the 4-L PEG agents with improved tolerance related to the decrease in volume consumed. Miralax is taken as 1 capful every 10 minutes in 8 oz of liquid until 2 L have been administered.1 This OTC agent is a cost-effective and easily accessible option for patients. The side effect profile and safety of Miralax is comparable to PEG, making it an effective option for colonoscopy preparation.

Patient Tips and Precautions

Patient counseling for bowel preparations is an important part of ensuring effective cleansing for the procedure. Some key elements that should be discussed with patients are ensuring that they complete the whole preparation, and to drink plenty of fluids.6 Drinking fluids—especially those with calories—ensures that the patient will not become dehydrated and/or experience a drop in their blood sugar as a result of their increased excretion. Other counseling tips that can help with tolerability of preparations include refrigerating the agent, drinking it through a straw, and adding flavoring, such as Crystal Light, to the preparation. When recommending the addition of flavoring, make sure to remind patients to avoid anything with red or purple dye, because these colors can be mistaken for blood during the colonoscopy.6

Addressing patient concerns in addition to other disease states and medications is important when choosing the best preparation agent. Each patient should be evaluated separately, and the risk versus benefit of each agent should be weighed. Cost and product accessibility are also important considerations when prescribing these agents. Discussing key features of the preparation prescribed is important to ensure an effective preparation has been chosen. Any side effects that can be expected, as well as administration and adherence counseling, are important factors that can ensure effective cleansing is achieved.


  1. World Laparoscopy Hospital. Guidelines for the bowel preparation prior to colonoscopy. Accessed December 15, 2014.
  2. ASGE Technology Committee, Mamula P, Adler DG, et al. Colonoscopy preparation. Gastrointest Endosc. 2009;69:1201-1209.
  3. Lexicomp. Hudson, OH: Lexi-Comp, Inc. Accessed 2014.
  4. Fingertip Formulary. Parsippany, NJ: Fingertip Formulary, LLC; 2013.
  5. Hayes A, Buffum M, Hughes J. Diabetic Colon Preparation for GI Procedure” VAMC San Francisco.
  6. Gastrointestinal Associates and Endoscopy Center. Bowel prep. Accessed December 15, 2014.
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