Side Effects of Colonoscopy Prep
Medical Procedures & SpecialtiesMedical ProceduresColonoscopy

6 Side Effects of Colonoscopy Prep You Should Know About

6 Side Effects of Colonoscopy Prep You Should Know About
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Updated on August 6, 2025
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No one looks forward to the prep the night before a colonoscopy. Unpleasant as it may be to take a strong laxative, the process is essential to empty your colon sufficiently for your doctor to see what's inside very clearly. Otherwise, they could miss something important, and you might need to go through the whole process all over again.

Fortunately, the prep has improved in recent years. 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

Side effects depend on which kind of colonoscopy prep solution you're prescribed.

There are two basic kinds. The first are called iso-osmotic preps. Their main ingredient is polyethylene glycol (PEG); brand names include GoLYTELY and MiraLAX. These are by far the safest preps — especially for people with kidney disease 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 in New York.

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: 4 liters (L).

The other basic types of preps are called hyperosmotic solutions. These may contain a variety of active ingredients. Hyperosmotic solutions can cause potentially dangerous fluid shifts, which means that people with underlying kidney, heart, or liver disease shouldn't drink them.

The main benefits of these preps are that you don't have to consume as much and they can be very inexpensive, Ivanina says.

Your level of health will be the main factor your doctor takes into account when deciding which type of prep solution to prescribe. So, tell your doctor about any medical conditions you have (including pregnancy), and let them know about any and all medicines you take. Some meds can affect the prep solution and cause a health risk.

Here are six possible 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 Medical Center 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 recommend drinking at least one 8-ounce (oz) 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 (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. 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

Pre-colonoscopy 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 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 in order 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
Everyday Health follows strict sourcing guidelines to ensure the accuracy of its content, outlined in our editorial policy. We use only trustworthy sources, including peer-reviewed studies, board-certified medical experts, patients with lived experience, and information from top institutions.
Resources
  1. Colonoscopy Prep. Cleveland Clinic. March 4, 2022.
  2. What Are the Best Types of Colonoscopy Prep? Colorectal Cancer Alliance. May 4, 2023.
  3. Medications to Avoid: GI Endoscopy Procedures. UConn Health.
  4. How to Prepare for Your Colonoscopy Using MiraLAX. Memorial Sloan Kettering Cancer Center. June 9, 2025.
  5. Fluid and Electrolyte Balance. MedlinePlus. May 16, 2024.
  6. Preparing for a Colonoscopy. Harvard Health Publishing. August 31, 2020.
  7. GoLYTELY Preparation for Colonoscopy. Columbia University.
  8. Risk of Kidney Problems After Colonoscopy. Fight Colorectal Cancer. September 4, 2022.
  9. Roswon H et al. Rare Case of PEG-Based Bowel Prep Severe Allergic Reaction. American Journal of Gastroenterology. October 2022.
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Waseem Ahmed, MD

Medical Reviewer

Waseem Ahmed, MD, is an assistant professor of medicine in the Karsh Division of Gastroenterology and Hepatology at Cedars-Sinai Medical Center in Los Angeles and serves as Director, Advanced Inflammatory Bowel Disease Fellowship and Education within the F. Widjaja Inflammatory Bowel Disease Institute.

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.

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