Preparing for a colonoscopy

The Family Health Guide

If you shudder at the thought of having a colonoscopy to check for hidden colon cancer, chances are it's the "prep" that's stoking your apprehension. It's certainly a major inconvenience: getting ready for the procedure takes much longer — an average of 16 hours, according to one study — than the three hours or so you'll spend at a medical center the day of your colonoscopy. But what's most off-putting is the purgative part: taking a powerful bowel-clearing substance and coping with the resulting diarrhea.

It's worth the hassle. Colonoscopy can spot small colon cancers while they are treatable and before they have spread to other parts of the body. It can also detect and remove polyps, small growths that can develop into colon cancer. Colon and rectal cancers (known together as colorectal cancers) are the third most common type of cancer in men and in women and the second leading cause of cancer-related deaths in the United States.

What's involved?

Emptying the contents of the colon is a key requirement for a successful colonoscopy. If the bowel prep isn't up to par, polyps and lesions can be missed; the colonoscopy may take longer (increasing the risk of complications); or the whole process may need to be repeated or rescheduled, meaning another round of bowel prep.

Different medical centers recommend different ways to prepare the bowel for a colonoscopy. There are two parts: diet and drinking bowel-cleaning liquids. Here are some of the main strategies.


A few days before the colonoscopy. Start a eating a low-fiber: no whole grains, nuts, seeds, dried fruit, or raw fruits or vegetables.

The day before the colonoscopy. Don't eat solid foods. Instead, consume only clear liquids like clear broth or bouillon, black coffee or tea, clear juice (apple, white grape), clear soft drinks or sports drinks, Jell-O, popsicles, etc.

The day of the colonoscopy. As for the previous day, clear liquid foods only. Don't eat or drink anything two hours before the procedure

Bowel prep

The afternoon or evening before the colonoscopy, drink a liquid that will trigger bowel-clearing diarrhea. The exact instructions depend on the bowel prep your doctor prefers, the time of your colonoscopy, and any prior experience you've had with colon preps (if one didn't work before, you'll likely be prescribed a different one).

Early bowel preps required drinking about one gallon of a bowel-cleaning liquid the day before the colonoscopy. A newer method is called split dosing. You take half the prep the night before the procedure and the other half about six hours before it. Many people prefer the split dose, and it may clean the colon more thoroughly.

Here are some common bowel preparations approved by the American Gastroenterological Association, American College of Gastroenterology, and American Society for Gastrointestinal Endoscopy. Contact your clinician to discuss the one that is best for you.

Common bowel preparations


What's involved


Polyethylene glycol
Brand names: Colyte, Golytely, Miralax, Nulytely, Trilyte

These preparations flood the gut with fluid, little of which is absorbed by the body. You generally need to drink about one gallon (4 liters), 8 ounces at a time, until the bowel's output runs clear or the solution is completely consumed. Polyethylene glycol preps can be used for split dosing.

The salty taste may be unpleasant, and the large fluid volume can cause nausea, vomiting, bloating, and cramping. An anti-nausea drug may be prescribed.

Oral sodium phosphate
Brand names: Fleet Phospho-soda EZ-Prep, Fleet Accu-prep

Two small doses of oral sodium phosphate solution diluted in at least 8 ounces of fluid are taken 10 to 12 hours apart. Each dose must be followed by 16 ounces or more of liquid. Adequate fluid intake is needed throughout the preparation and after the colonoscopy.

Oral sodium phosphate draws water into the gut. It is effective, but can cause dehydration and electrolyte imbalances. It may also damage the kidneys.

Oral sodium phosphate tablets
Brand names: Visicol, OsmoPrep

Twenty tablets are taken the evening before the colonoscopy, 4 at a time with 8 ounces of clear liquid every 15 minutes. Depending on the preparation, 12 to 20 more are taken the same way a few hours before the procedure.

Like oral sodium phosphate solutions, the tables can cause dehydration and electrolyte imbalances, and can harm the kidneys.

Magnesium citrate
Many over-the-counter brands

Available over the counter, magnesium citrate works by pulling water into the small intestine. When used in split dosing, you drink 15 ounces of magnesium citrate the evening before the colonoscopy and 15 ounces six hours before the procedure. Some medical centers combine magnesium citrate with the laxative bisacodyl (Dulcolax),

Magnesium citrate shouldn't be used by individuals with kidney disease.

Tips for a successful colonoscopy prep

Preparing for a colonoscopy may be uncomfortable and time-consuming, but it needn't be an ordeal. Here are some things you can do to help it go as smoothly and comfortably as possible:

Make sure you receive your colonoscopy prep instructions well before your procedure date, and read them completely as soon as you get them. This is the time to call your clinician with any questions and to buy the bowel prep she or he has prescribed. Pick up some medicated wipes (for example, Tucks or adult wet wipes with aloe and vitamin E) and a skin-soothing product such as Vaseline or Desitin — you're going to be experiencing high-volume, high-velocity diarrhea.

Arrange for the time and privacy you need to complete the prep with as little stress as possible. Clear your schedule, and be at home on time to start your prep. If you have children or aging parents who need attention, have someone else be available to them while you're indisposed.

Water can get boring, so keep a variety of clear liquids on hand. On the day before your colonoscopy — when you're restricted to clear liquids — you can have popsicles, Jell-O, clear broth, coffee or tea (without milk or creamer), soft drinks, Italian ice, or Gatorade. But take nothing with red, blue, or purple dye. Drink extra liquids before, during, and after your bowel prep (usually until a few hours before your procedure), as well as after your colonoscopy.

To make a bad-tasting liquid prep like PEG or magnesium citrate easier to swallow, try one or more of the following if it doesn't come flavored: add some Crystal Light or Kool-Aid powder (again, not red, blue, or purple); add some ginger or lime; drink it chilled; drink it through a straw placed far back on your tongue; hold your nose and drink it as quickly as possible; quickly suck on a lemon slice after you finish each glass; hold a lemon or lime under your nose while you drink; suck on a hard candy after each glass.

Wear loose clothing, and stay near the bathroom. Better yet, once the preparation starts to work, stay in the bathroom — because when the urge hits, it's hard to hold back. Consider setting up shop near the toilet with music, your laptop, magazines, or books.