Preparing for a colonoscopy isn’t anyone’s favorite task, but it’s important to empty your colon sufficiently so your doctor can see what's inside very clearly.
While it can be unpleasant to take a strong laxative, skipping colonoscopy prep may cause your doctor to miss something important, or you may need to repeat the process. Plus, some newer prep solutions taste better, and you don’t have to drink as much of them or all at once.
There are two main types of colonoscopy prep solutions: iso-osmotic preps and hyperosmotic solutions.
Iso-osmotic preps, like GoLYTELY and MiraLAX, are the safest preps for people with kidney disease or heart disease because they mimic the body's chemical composition and don't disrupt fluid and electrolyte balances. But you do have to drink a lot of them, and they’re the least palatable of the two kinds of preps, says Elena Ivanina, DO, a gastroenterologist in New York City.
With hyperosmotic solutions, you don’t have to drink as much and they’re less expensive. But they’re not right for people with underlying kidney, heart, or liver disease.
Your doctor will take into account your level of health, medical conditions, and any medications you’re taking when deciding which type of prep solution to prescribe. They can also walk you through the process and advise you on potential 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, a clear sports drink, 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 Medical Center in Kansas City. “If they get dehydrated too much, we start seeing kidney problems.”
Experts recommend drinking at least one 8-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 with the iso-osmotic preps such as GoLYTELY. Hyperosmotic preps containing sodium phosphate 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 antinausea medication.”
Aside from taking stomach-soothing medication, there are other ways to quell the queasy feeling. Some 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 suck on a lemon or hard candy after drinking each glass.
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 have a little bit of stomach pain, cramping, and bloating when prepping for a colonoscopy. But if the pain becomes severe, 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.
5. Kidney Problems
Precolonoscopy dehydration can cause kidney issues. That's 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,” Ashcraft says.
6. Allergic Reactions
Official case reports of allergic reactions to colonoscopy preps are extremely rare.
People with heart failure or liver or kidney damage are already very sensitive to their fluid balance, says Dr. Ivanina. They should 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, people with electrolyte disorders, kidney disease, dehydration, or heart failure should avoid them for that reason as well, she says.
Regardless of what kind of prep your doctor prescribes, they should alert you to the potential side effects. If you're concerned about the seriousness of any prep side effects that you notice, call your doctor right away — or go to the emergency room.
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.
The Takeaway
Preparing for a colonoscopy involves taking a strong laxative the night before to clear the bowels before the procedure.
There are several different types of bowel prep formulations, some of which are more suitable for people with certain health conditions.
Side effects of bowel prep can include nausea, dehydration, and stomach cramps. More severe side effects are possible but rare.
Talk with your doctor if you have any concerns before you start bowel prep.
EDITORIAL SOURCES
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He received his undergraduate degree from the University of Michigan and attended medical school at Indiana University. He then completed an internal medicine residency at New York University, followed by a fellowship in gastroenterology and hepatology at Indiana University, and an advanced fellowship in inflammatory bowel disease at the Jill Roberts Center for Inflammatory Bowel Disease at New York-Presbyterian Hospital/Weill Cornell Medicine. Prior to his current role, Dr. Ahmed served as an assistant professor of medicine within the Crohn’s and Colitis Center at the University of Colorado from 2021-2024.
Dr. Ahmed is passionate about providing innovative, comprehensive, and compassionate care for all patients with inflammatory bowel disease (IBD). His research interests include IBD medical education for patients, providers, and trainees; clinical trials; acute severe ulcerative colitis; and the use of combined advanced targeted therapy in high-risk IBD.
He enjoys spending time with his wife and dog, is an avid follower of professional tennis, and enjoys fine dining.
Dr. Amanda Gardner is a freelance health reporter and community artist based in Sandia Park, New Mexico, who works with marginalized, homeless, and incarcerated populations. She coauthored the Prison Arts Resource Project, an evidence-based bibliography of research into arts programming in correctional facilities. Her health journalism appears in major publications — BuzzFeed, Reader’s Digest, Health.com, CNN.com — while her academic work focuses on community arts and social justice.
