6 Side Effects of Colonoscopy Prep You Should Know About

No one looks forward to the prep that has to be done before a colonoscopy: taking heavy laxatives, drinking gallons of water and, most notoriously of all, consuming the colon prep solution prescribed by the doctor.

The most common side effect of colonoscopy prep is dehydration, which can cause dizziness. (Image: Getty Images)

Unpleasant as it may be, though, the process is essential in order to get your colon empty enough for your doctor to see what's inside very clearly. Otherwise, he or she could miss something important — and you might need to go through the whole process all over again.

The good news is that colonoscopy prep has improved immensely in recent years. As experts at the Cleveland Clinic have noted, some newer prep solutions not only taste better, but you don't have to drink as much of them. And you don't always have to drink them all at once — you can consume some the night before your procedure, and the rest the morning of.

Types of Prep Solutions

There are two basic kinds of colonoscopy prep solutions. The first are called iso-osmotic preps. Their main ingredient is polyethylene glycol, aka PEG; brand names include GoLYTELY and MiraLAX. These are by far the safest preps — especially for people with kidney or heart disease — because they mimic the body's chemical composition and don't disrupt fluid and electrolyte balances.

"[Iso-osmotic preps] have the same electrolytes as your own body, so [drinking them] is not a big shock to the system," says Elena Ivanina, DO, a gastroenterologist with Lenox Hill Hospital in New York City.

The bad news? They're the least palatable of the two kinds of prep solutions, says Dr. Ivanina. And you have to drink a lot (four liters!).

The other type of preps are called hyperosmotic solutions. These may contain a variety of active ingredients (such as sodium phosphate, sodium sulfate and magnesium citrate) and can cause potentially dangerous fluid shifts, which means that people with underlying kidney, heart or liver disease shouldn't drink them. In fact, all colonoscopy patients must undergo a kidney function test before taking hyperosmotic preparations. Their main benefits, according to Dr. Ivanina? You don't have to consume as much, and they can be very inexpensive.

Prep side effects depend on which kind of solution you're prescribed. According to the American Society of Gastrointestinal Endoscopy, your level of health will be the main determinant, so tell your doctor about any medical conditions you have, especially if you are pregnant, have high blood pressure, have a history of bowel obstruction or have heart, kidney or liver disease. Your doctor will also need to be informed as to any and all medications you're on, particularly blood thinners, insulin, arthritis medications and products containing aspirin. (These and other meds can interact with the prep and cause a health risk, according to experts in the division of gastroenterology and hepatology at UConn Health.)

Here are some of the more typical colonoscopy prep side effects:

1. Dehydration

Dehydration is probably the most common side effect of colonoscopy prep, because so much fluid is lost via the bowel cleanse. That's why doctors advise drinking as much liquid as you can during and after the prep, be it in the form of water, clear Gatorade, white grape juice, bouillon or other clear liquids.

"The more fluids patients drink, the more that will counteract the dehydration," says John Ashcraft, DO, chief of colon and rectal surgery at the University of Kansas Health System in Kansas City. "If they get dehydrated too much, we start seeing kidney problems."

Signs of dehydration include feeling really thirsty, having dark urine, not peeing very often and feeling dizzy. Experts at the Memorial Sloan Kettering Cancer Center recommend drinking at least one eight-ounce glass of water (or other clear liquid) every hour, unless you're asleep. Staying hydrated also keeps your electrolytes in balance, which is critical for the healthy functioning of muscles and nerves.

2. Nausea

Nausea and vomiting are other potential side effects of colonoscopy prep, especially the iso-osmotic preps such as GoLYTELY. Hyperosmotic preps containing sodium phosphate (NaP) can also cause nausea.

"A lot of patients have problems with nausea," Dr. Ashcraft says. "When I prescribe the prep, part of the package is a prescription for anti-nausea medication."

Aside from taking stomach-soothing medication, there are other ways to quell the queasy feeling. Harvard Medical School experts recommend adding ginger, lime or drink-mix powder (as long as it's not red, blue or purple) to the prep liquid. You can also chill the solution before drinking it, and/or suck on a lemon or hard candy after drinking each glass.

Research from the American College of Surgeons recommends rinsing your mouth and taking a half-hour break after drinking the prep if you start to feel nauseated.

If you do end up vomiting, take a breather, then try to finish the rest of the prep solution. When you show up for your colonoscopy appointment, let the doctor know that you had challenges and weren't able to keep down all of the solution.

3. Abdominal Pain

Given the volume of fluid you're taking in, it makes sense that you might experience a little bit of stomach pain, cramping and/or bloating when prepping for a colonoscopy. If the pain becomes severe, though, call your doctor.

4. Chills

Some people get the chills after drinking certain prep solutions. This is nothing to worry about. It's simply because you're drinking so much cold fluid, according to experts at Columbia University.

5. Kidney Problems

Pre-colonoscopy dehydration can cause kidney issues, because both dehydration and its accompanying electrolyte imbalance can contribute to kidney trouble. If you already have underlying kidney problems, your doctor will prescribe a very specific colonoscopy prep solution.

"We also encourage these patients to watch their fluid status, and we are in constant contact with their renal specialist," Dr. Ashcraft says.

6. Allergic Reactions

Official case reports of allergic reactions to colonoscopy preps are extremely rare. A 2011 article in the journal Military Medicine reported only a handful of cases.

People with heart failure or liver or kidney damage "are already very sensitive with their fluid balance," notes Dr. Ivanina. They should therefore avoid hyperosmotic colonoscopy preps, which can aggravate those conditions. Also, hyperosmotic preps that contain any kind of phosphate have been linked with seizures in rare cases, so patients with electrolyte disorders, kidney disease, dehydration and heart failure should avoid them for that reason as well, she says.

Regardless of what kind of prep your doctor prescribes, he or she should alert you as to the potential side effects. If you're concerned about the seriousness of any prep side effects you do experience, call your doctor right away — or go to the ER.

Colonoscopy preps are often unpleasant, but they shouldn't harm your health. Make sure that you and your doctor have a conversation about all of your existing medical conditions and medications before you start the colonoscopy process.

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