How to prep for a successful colonoscopy

The bowel preparation phase, better known as "bowel prep," is considered the most unpleasant part of the colonoscopy process, but it's also crucial. A clean bowel aids the doctor in identifying and removing polyps, small growths that can develop into cancer.

"If the bowel preparation is inadequate, you'll have to repeat the entire process - prep and colonoscopy," says gastroenterologist Dr. Brian Jacobson, director of program development for the Division of Gastroenterology at Harvard-affiliated Massachusetts General Hospital.

How bowel prep works

The bowel prep is typically done over a period of one or two days before the colonoscopy, or in some cases on the same day. It involves drinking up to 4 liters of laxative solution (or taking tablets) to flush the colon clean of stool and food particles. A day or two before starting the prep, you'll switch to a low-fiber diet and then transition to a clear liquid diet until after the colonoscopy is completed.

How to do a successful prep

There are several ways in which someone might fail to complete a suitable bowel prep. Some people get confused about the doctor's instructions. Some have trouble taking the laxative. And some struggle with the dietary requirements.

With the aim of ensuring more successful colonoscopies, the U.S. Multi-Society Task Force on Colorectal Cancer recently updated its bowel prep guidelines for gastroenterologists. "They offer multiple ways gastroenterologists can individualize prep plans and better guide their patients through the process," says Dr. Jacobson. The update appeared in the April 2025 issue of the journal Gastroenterology.

Here are the guidelines' main takeaways.

Instructions. Patients should receive both verbal and written instructions about all aspects of the process. "Patients do better bowel prep when detailed instructions are well communicated," says Dr. Jacobson. "This helps to avoid confusion and ensure they comply with all the prep requirements." Call your gastroenterologist's office if the instructions are unclear.

Laxative choice. The guidelines didn't recommend specific laxatives but offered a detailed list of commonly used ones (solutions and tablets), along with their side effects and contraindications. The doctor's choice of laxative should take into consideration the patient's preference, the cost burden, and how easily they can take the formulation. "People who struggle drinking the solution may prefer tablets, even though it involves taking 24 pills," says Dr. Jacobson.

Risk factors. Certain conditions and medications increase a person's risk for a poor bowel prep, including cirrhosis of the liver, Parkinson's disease, dementia, diabetes, overweight, and antidepressant use. "If any of these issues apply, your doctor can adjust the prep regimen as needed," says Dr. Jacobson.

Laxative quantity. Patients at low risk for having an inadequate prep can drink a 2-liter solution instead of a 4-liter solution. "Research has found that for these people, the lower amount is just as effective, and it reduces the risk for side effects like cramping and nausea," says Dr. Jacobson.

Timing the doses. Individuals who drink the solution in two portions (known as a split-dose regimen) should take the second portion four to six hours before the colonoscopy. Same-day preparation is acceptable for people undergoing an afternoon colonoscopy, but not for those getting the procedure in the morning.

High-risk help. The guidelines suggest a more advanced regimen for people at high risk of having an inadequate prep. "That might mean more restrictive diet changes in the days leading up to the colonoscopy, use of a larger volume of laxative, and possibly the addition of another medication to stimulate bowel emptying," says Dr. Jacobson.

Foods and meals. The guidelines provided sample menus and examples of low-fiber and low-residue foods (foods that are easier to digest and leave less waste in the colon) that have been shown to help with bowel preparation.

When prep goes wrong

Finally, the guidelines offer guidance to doctors on how to deal with a patient who hasn't prepped properly. "If an individual thinks their prep may be inadequate just based on their body's response to the laxative, this doesn't mean the colonoscopy can't still be performed," says Jacobson. The guidelines recommend that the gastroenterologist examine the lower region of the colon and then decide whether to proceed. If the procedure can't be completed, timing for another try depends on why the colonoscopy was originally scheduled.

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