How to Know When Bowel Prep Is Complete

Bowel prep is something you must do to clean out your colon before getting a colonoscopy. A colonoscopy is an invasive medical test that examines the colon (large intestine) and rectum. During the procedure, the healthcare provider uses a flexible camera called a scope to search for visible problems like bleeding, polyps (small growths), or signs of cancer.

Although it's not a fun experience, completing a bowel preparation doesn't hurt. In other words, there's no reason to run for the hills when it's time to prepare for this all-important screening.

This article will discuss how to know when your bowel prep is done.

successful colonoscopy bowel prep

Verywell / Jessica Olah

Things to Do Before You Begin

Before you begin your bowel prep, tell your healthcare provider if you have any heart, kidney, or liver problems. There are many different bowel prep options, and some medical conditions require specific types of bowel prep.

It's important to make sure the healthcare provider performing your colonoscopy is familiar with your medical history for other reasons too. Some medications, such as blood thinners, can increase the risk of complications from bowel prep.

Review Your Instructions

When you're having your bowel prep, make sure you stay close to home. That's because you'll be using the toilet many, many times to clean out all the poop before the process is through.

Your gastroenterologist (a healthcare provider who treats conditions related to the digestive system) needs every bit of stool (poop) cleaned out of your colon in order to do the procedure. A proper bowel prep will involve more than two or three bowel movements so your healthcare provider can visualize abnormal tissue that would otherwise be hidden by stool.

All gastroenterologists follow the cleansing guidelines approved by the American Gastroenterological Association, American College of Gastroenterology, and American Society for Gastrointestinal Endoscopy.

But each healthcare provider may order the bowel prep a little differently. The instructions can even vary based on what time you'll be getting the test. What a healthcare provider tells your friend to do for a bowel prep might not be the same instructions you receive.

Before your healthcare provider gives you directions on how to complete your bowel prep, they will consider:

  • Your health
  • What you may or may not be able to tolerate
  • What has worked best in the past

There are some types of preps that you shouldn't do if you have certain medical conditions. A healthcare provider may recommend one bowel prep for you and another for someone else.

Getting Started

Most bowel preparations begin with a liquid for you to drink or pills for you to swallow. You might start to see effects as soon as 30 minutes to an hour after your first glass of solution or your first pill. Your first bowel movements will most likely be a combination of firm, semi-firm, or loose brown stools. 

You will be on a clear liquid diet, and you should continue to drink plenty of clear liquids and keep yourself hydrated. Avoid commercial drinks with purple or red dye. These dyes can interfere with your colonoscopy results.

Staying Hydrated

Staying hydrated is crucial. Your body needs enough water to function properly. Many of the negative events related to a colonoscopy occur because of dehydration during the procedure. Dehydration can easily happen with bowel prep products containing sodium phosphate.

You might notice some uncomfortable, but not painful, side effects of the bowel prep. Stomach cramping and gas are completely normal. Laying a clean, damp cloth on your stomach can help ease discomfort. A small bit of activity—such as a walk around the house—may also help relieve uncomfortable side effects.

Following each step of your healthcare provider's bowel prep instructions is very important. Sometimes it may seem like you're just repeating steps, such as using an enema after having several bowel movements. But these steps are necessary.

Finishing the Bowel Prep

When your bowel movements contain only brown liquids, this means that you are almost finished with your bowel prep. Your stools should eventually end up as a clear or yellowish liquid. If there is any cloudiness to your liquid stool, your bowel prep is not complete.

Your stools may become clear, liquid, and yellowish before you complete your entire prep. It's important to finish the prep anyway. You could still have stool higher in your colon that will come out later. Completing all steps of your prep offers you the best chance of having a clear colon for your colonoscopy (and not having to repeat the test).

Just because your stools are clear, liquid, and yellowish doesn't mean your bowel prep is over. It's important to follow all of the bowel prep steps.

Why Bowel Prep Matters

According to research published in the journal Gastroenterology, up to 25% of colonoscopies are canceled due to an unsuccessful bowel prep. Deciding to cancel your procedure is not an easy decision for a healthcare provider to make if you are already sedated and the practitioner has already started the procedure. The healthcare provider can only continue with your colonoscopy if your colon is completely clean.

If you didn't finish your prep or your bowel movements aren't reduced to clear liquids, contact your healthcare provider's office so you can reschedule your procedure.

Sometimes you may have the exam but receive inadequate results if there is still stool in your colon. This often happens when people skip one of the steps of the prep because they believe they are done. It can also happen because they don't think all of the steps are necessary.


Before you get a colonoscopy, it's important that you follow the bowel prep directions given to you so that you know when the prep is done. Your stools should be clear, yellowish liquid near the end of the prep.

But don't stop then: Make sure to complete all the steps in the directions. Closely following these instructions will help ensure the colonoscopy is effective.

A Word From Verywell

Preparing for a colonoscopy isn't exactly fun, but it's an important exam that can help you live a long, healthy life. By following your healthcare provider's bowel prep directions, you will prepare for the colonoscopy effectively so you can get reliable answers from this test.

Frequently Asked Questions

  • Will a colonoscopy prep keep you up all night?

    It may, but it doesn’t usually. You make wake up once or twice to go to the bathroom, but it shouldn't keep you up all night.

    Depending on your doctor's instructions, you may need to wake in the middle of the night to take a second dose of laxatives. Known as a split-dose bowel prep, the second dose should be taken at least four to six hours before the scheduled procedure.

  • How long does it take a colonoscopy prep to clear bowels completely?

    It can take 12 to 16 hours for the bowels to clear completely in preparation for a colonoscopy. Eating a low-residue, soft diet for a day or more before starting the prep can help make it easier and faster. A low-residue diet limits high-fiber foods, such as whole grains, nuts, seeds, fresh or dried fruit, and vegetables. 

  • Do you need an enema before a colonoscopy?

    You may. If the laxatives do not clear your bowels completely, your doctor may instruct you to give yourself an enema in the morning before your colonoscopy. The enema should be given at least two hours before your scheduled arrival time.

    An enema typically begins working in about five minutes but may take up to an hour to work back out. If you have a long drive to the testing center, you can take the enema two hours before your need to leave for the appointment.

4 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Rutherford CC, Calderwood AH. Update on bowel preparation for colonoscopy. Curr Treat Options Gastroenterol. 2018;16(1):165-181. doi:10.1007/s11938-018-0165-3

  2. Harvard Health. Preparing for a colonoscopy.

  3. American Society for Gastrointestinal Endoscopy. Understanding bowel preparation.

  4. Johnson DA, Barkun AN, Cohen LB, et al. Optimizing adequacy of bowel cleansing for colonoscopy: recommendations from the US multi-society task force on colorectal cancer. Gastroenterology. 2014;147(4):903-24. doi:10.1053/j.gastro.2014.07.002

By Julie Wilkinson, BSN, RN
Julie Wilkinson is a registered nurse and book author who has worked in both palliative care and critical care.