Harvard Women's Health Watch

Preparing for a colonoscopy

Worse than the colonoscopy itself? Probably. Here's how to make this unpleasant but essential task go more smoothly.

If you shudder at the thought of having a colon cancer–screening colonoscopy, 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 — according to one study, 16.5 hours, on average — than the time you'll spend at a medical center the day of your colonoscopy (usually no more than three hours). But what's most off-putting is the purgative part — taking a powerful bowel-clearing substance and coping with the resulting diarrhea.

Why colonoscopy?

During a colonoscopy, a specially trained clinician (usually a gastroenterologist) examines the lining of the colon (large intestine) with a lighted scope (colonoscope) looking for cancerous lesions and polyps — growths on the inside surface of the colon that sometimes turn into cancer. There are other screening methods — fecal occult blood testing, flexible sigmoidoscopy, double-contrast barium enema, CT colonography (virtual colonoscopy), and a stool test for DNA shed by colorectal cancers — but traditional colonoscopy is preferred. It finds more polyps and lesions than the other methods, and polyps can be removed and biopsies taken immediately during the procedure. Colonoscopy also checks the entire colon. This is especially important for women, because they're more likely to have polyps or lesions deeper in the colon — in particular, in the ascending colon, the area at the furthest reach of the colonoscope. (Sigmoidoscopy, another procedure that uses a lighted scope, only examines the first third of the colon.)

Although no studies have proved it, there's some reason to believe that bowel prep is harder for women than for men. "Women are more likely to be constipated," says Harvard Medical School's Dr. Jacqueline Wolf, a gastroenterologist at Beth Israel Deaconess Medical Center in Boston. "Therefore their motility might not be quite as good, so it may be a little harder to get them cleaned out." Women are also more likely than men to have irritable bowel syndrome, which can cause gas, bloating, and abdominal pain or spasm. "You can imagine that it could cause problems with the prep," says Dr. Wolf.

What's involved?

Emptying the contents of the colon is the first 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.

You'll need to stop eating solid foods the day before the colonoscopy, and starting the evening before the procedure, you must take a substance that triggers bowel-clearing diarrhea. Exact instructions will depend on the bowel prep prescribed, 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).

The two most widely used bowel preps are polyethylene glycol (PEG) and oral sodium phosphate. Both require the rapid ingestion of large amounts of fluids. Diarrhea usually starts within one to four hours of starting to drink the preparation. The PEG solution is more difficult to swallow — literally. It's salty and you must drink a gallon of it in less than three hours. PEG can cause bloating, nausea, and vomiting, which can result in incomplete emptying of the bowel. An antinausea medication may be prescribed. Another approach is to use half the amount of PEG solution along with a laxative, such as bisacodyl (Dulcolax) or magnesium citrate.

Oral sodium phosphate solution is also briny, but some people find it easier to tolerate than PEG. It's taken in two small doses, each mixed with 8 ounces of a clear liquid such as Sprite or ginger ale, and followed by two or more 8-ounce glasses of clear liquids. The latest answer to the palatability problem is Visicol and OsmoPrep — oral sodium phosphate in tablet form. The tablets are taken four at a time, every 15 minutes, with eight ounces of a clear liquid — 20 tablets the night before the colonoscopy, and 12 to 20 tablets a few hours before the procedure.

Oral sodium phosphate works by drawing water into the bowel to flush out waste. This can lead to dehydration and electrolyte imbalances, so it's important to stay well hydrated throughout the bowel prep; Gatorade or Powerade are especially helpful.

The FDA added a warning to Visicol and OsmoPrep in late 2008 after the agency received reports of acute phosphate nephropathy, a type of kidney injury, in people using these bowel cleansers. The warning states that oral sodium phosphate products should not be used with other laxatives that contain sodium phosphate, and that certain groups of people seem to be more likely to have kidney trouble with these products. Therefore, they should be used with caution in people over age 55; people suffering from dehydration, kidney disease, active colitis, or delayed bowel emptying; and people taking medicines that affect kidney function, such as diuretics, ACE inhibitors, angiotensin receptor blockers, and perhaps the pain killers known as non-steroidal antiinflammatory drugs (NSAIDs).

Common bowel preparations


What's involved



On the day preceding and up to a few hours before the procedure, consume clear liquids only: broths, soft drinks, tea and coffee (with no milk or creamer), clear juices, Jell-O; but no red, blue, or purple coloring. You may also be asked to avoid nuts, seeds, hulls, skins, or other insoluble fiber for a day or more before beginning clear liquids.

Used in conjunction with colon-cleansing preparations, never as the sole bowel preparation. Recommendations vary and may depend on individual situation, clinician preference, or both.

Polyethylene glycol (PEG)

Brand names: Golytely, Colyte, Nulytely, Trilyte, Halflytely

This is an electrolyte-balanced solution consumed in large amounts (4 liters, or about 1 gallon) the evening before a colonoscopy. Patients drink 8 ounces every 10 minutes until the bowel's output runs clear or the solution is completely consumed. In another approach, the dose is divided into 3 liters the night before and 1 liter the morning of the colonoscopy. A newer preparation (for example, Halflytely) uses 2 liters (about 2 quarts) of PEG solution taken in conjunction with a laxative (bisacodyl or magnesium citrate).

PEG solutions work by pushing a large volume of fluid through the bowel to force out waste. They cause no significant electrolyte shifts, so they are considered safer than OSP solutions (see below). The salty taste may be unpalatable, and the large fluid volume can cause nausea, vomiting, bloating, and cramping. An antinausea drug may be prescribed.

Oral sodium phosphate (OSP) solution

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

Two small doses of OSP 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.

OSP draws water into the gut to promote cleansing. It's as effective as PEG and easier to tolerate, but it can cause dehydration, electrolyte imbalances, and, in rare instances, kidney damage.

Oral sodium phosphate (OSP) 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. Bisacodyl may be added.

OSP tablets are as effective as liquid OSP or PEG and may be more tolerable. They may cause dehydration, electrolyte imbalances, and kidney damage in some people.


Magnesium citrate


Either laxative may be used with lower-volume (2 liters) PEG. Both laxatives may be used in combination with a restricted diet. Magnesium citrate may be used alone.

There are rare reports of ischemic colitis (loss of blood supply to the colon) associated with bisacodyl use.

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. You'll need to stop certain medications and foods up to a week ahead of time. This is also 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.

  • If you're prone to constipation, consult your clinician about using something extra — a laxative such as Correctol, Senokot, or castor oil — to get things going two or three days before the prep. You can also try a laxative herbal tea (for example, Smooth Move or Get Regular). Dr. Wolf suggests that you discontinue potentially constipating supplements such as calcium, iron, and multivitamins containing either one for a week or so before your colonoscopy. Exercise and drinking plenty of fluids can also help.

  • 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 easier to swallow, try one or more of the following: if the PEG 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.

  • After your bowel has emptied, take a warm bath or shower. Put on some comfortable nightclothes, and get into bed. If you feel chilly or your IBS is acting up, place a heating pad or hot-water bottle on your stomach.