Getting the colon free of stool and ready for colonoscopy is a major challenge.  In some studies fully one third of people say that the prep is the key barrier to their being screened for colorectal cancer.

Gastroenterologists offer a variety of strong laxatives designed to completely cleanse the colon from a gallon of polyethylene glycol solution  (PEG) to a much smaller volume of sodium phosphate liquid (Eleet Phospho-soda®).  In recent years sodium phosphate has been available in tablet form (Visicol® and OsmoPrep®). 

The Halflytely® bowel preparation reduces the amount of PEG solution to two liters and adds four bisacodyl laxative tablets.

In order to compare the  safety and colon-cleansing effectiveness of sodium phosphate tablets to the two-liter PEG prep, researchers randomized 481 adults having a colonoscopy to use either

  • 32 sodium phosphate tablets with clear liquid of their choice
  • 2 liters of PEG solution plus 4 bisacodyl tablets

Colonoscopists did not know which prep their patients had received and rated cleansing from excellent to inadequate both overall and for the ascending colon.  The sodium phosophate tablets were significantly better than the PEG solution for cleaning stool in both assessments.

Patients who took sodium phosphate tablets were less likely to have gastrointestinal symptoms including bloating, abdominal pain, or vomiting than those who used PEG plus the bisacodyl: (64 percent versus 79 percent.) Overall, adverse events were less likely for the sodium phosphate tablets than for the PEG liquid (66 percent versus 82 percent.)

Both groups had changes in laboratory tests, but fluctuations were more common with sodium phosphate tablets especially for phosphorous, sodium, and potassium.

Led by John F. Johanson, M.D., the team concluded,

The colon-cleansing efficacy of the new 32-tablet NaP dosing regimen in this study was found to be significantly better than the 2L PEG solution plus bisacodyl tablets regimen. The 32-tablet NaP dosing regimen was associated with fewer adverse events. As expected electrolyte shifts were more common and of greater magnitude in the NaP group compared with the PEG plus bisacodyl group; however, both treatment groups demonstrated significant changes in electrolytes and creatinine.

SOURCE:  Johanson et. al., American Journal of Gastroenterology, early online articles, June 15, 2007.


Patients preparing for a colonoscopy can feel comfortable that sodium phosphate tablets will clean the colon as well as or better than a two liter container of PEG solution.

They may have less bloating, cramping, and vomiting with the tablets.

However, patients with kidney or cardiac problems will want to discuss the choice of colonoscopy prep carefully with their doctors since there is a greater change in body electrolyte balance with tablets.

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