What’s The Best Prep?


You probably know how awful bowel prep is. You also might know that there are different kinds of bowel prep, and the regimen can vary from doctor to doctor. So what is the best bowel prep?

Bowel prep — for those blissfully unaware — is the process of cleansing out your large intestine. It is sometimes done before surgeries, but most often before a colonoscopy. Bowel prep is important in colonoscopy so that the colonoscope can record accurate pictures of the internal lining of the gut.

Usually, bowel prep involves fasting for a day — drinking only clear liquids, no red Jell-o — and then taking a laxative to clean out the gut. The laxative typically works by osmosis to force the gut to secrete water, which rinses your insides clean. Osmosis is also how table salt kills slugs, by the way.

Beyond the basic chemistry, there are different kinds of prep and different ways to take it. The four main chemicals used for bowel prep are:

  • Polyethylene glycol (PEG): the main ingredient in products such as Colyte, GaviLyte, Golytely, MiraLax, Klean-prep, MoviPrep, and others.
  • Sodium picosulfate: Dulcolax, Prepopik, etc.
  • Sodium phosphate: Phosphoral, Osmoprep (the pill-based bowel prep). Sodium phosphate was also in Fleet’s Phosphosoda, which was removed from the US market because it was found to cause kidney damage in a small number of patients.
  • Magnesium citrate, which is available in generic products.

In terms of what you take, PEG is obviously the most popular product. Evidence tends to show it slightly outperforms other forms of prep. One study found PEG superior to sodium phosphate; another study found that PEG + sodium phosphate is better than PEG alone, and either was better than magnesium citrate. However, yet another study suggests that the evidence comparing PEG and sodium phosphate is inconclusive, but sodium phosphate tablets perform best of all.

When you take the prep also can make a difference: while many of us are used to taking the whole dose of prep the night before a colonscopy, there are also split-dose and same-day regimens. Split-dose means taking half the night before, and half the morning of the exam. Same-day is taking it all the same-day, for an afternoon procedure.

A review study shows that split-dose PEG is much better than night-before, but a more recent study in which 4 liters split-dose of PEG was tested against biscadoyl followed by 2 liters PEG split-dose showed the latter was “as effective but better tolerated“. In a test of split-dose versus same-day prep, the former was slightly better statistically; but in practical terms, same-day prep was just as good, and much easier on patients. An editorial in the same journal says ‘everybody wins’ with same-day prep.

One of the problems with this research is that the ‘quality’ of bowel prep is somewhat subjective. What counts as ‘good’ depends on who you ask: there are at least three different scales to measure bowel prep quality. Also, the groups of patients the studies looked at vary a great deal; what works for a 75-year-old patient with no history of bowel disease may not work for a 17-year-old in a bad UC flare.

What most of the studies agree on is that the best prep is one that the patient finishes. Here’s one study that  gives up on naming a winner, instead concluding that “by taking into account individual patient characteristics, opting for a particular regimen could increase the likelihood of achieving a cleaner colon.” Another study suggests that whether patients find prep tolerable is an important factor in polyp detection.

Overall, split-dose 4 liter PEG seems to have the most evidence going for it. But it is important that you take a prep that you can tolerate: if 4 liters of PEG is too much (it was for me), you can ask your doctor about some of the other regimens discussed above. The best prep is one you will actually complete.

Fun bonus: there is a raging debate among gastroenterologists and anesthesiologists over how long patients should wait after their last dose of bowel prep before receiving sedation for colonoscopy: 96% of anesthesiologists think it should be more than 2 hours, while 26% of gastros think it should be that long. Somehow this debate hinges on the fact that anesthesiologists don’t consider bowel prep a ‘clear liquid’. So if your colonoscopy is a long, long time after your last dose of prep, it might not be your gastro’s fault.

Photo of a bowel prep jug by author Duncan Cross. For the record I did not finish the whole thing – not even half. But I have a lot less large intestine than most people.

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