How to Do a Bowel Prep for Colonoscopy or Surgery

Bowel preparation, commonly known as bowel prep, is the process of removing feces from the colon prior to a medical or surgical procedure. It is important to clean the colon of all stool, food particles, and any other residues that may be clinging to tissue surfaces.

If you are having surgery on or near the colon, even a tiny amount of stool can pose a risk of infection if the bowel is accidentally nicked. Stool can also get in the way of certain imaging procedures, such as a colonoscopy, or make it difficult to operate on adjacent tissues, such as the uterus if the colon is distended.

If bowel prep is needed, your healthcare provider will provide you with detailed instructions that you will need to follow exactly. Here is a step-by-step guideline of what to expect:


Medication Changes (7 Days Prior)

Man talking to his doctor
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Bowel prep procedures vary between healthcare providers and procedures. Generally speaking, the guidelines are similar and always start with changes in medication routines and a restriction on what you can and cannot eat.

Starting with medications, some will need to be stopped as much as seven days in advance of the procedure. Chief among them are blood thinners like Coumadin (warfarin), Plavix (clopidogrel), and Lovenox (enoxaparin). Other drugs that may need to be stopped or adjusted include:

  • Antidiarrheals like Imodium (loperamide), Lomotil (atropine/diphenoxylate), and Pepto Bismol (bismuth subsalicylate), which slow intestinal motility
  • Nonsteroidal anti-inflammatory drugs (NSAIDs) like aspirin, Advil (ibuprofen), and Aleve or Motrin (naproxen), which can increase the risk of bleeding
  • Fiber supplements like Metamucil, Citrucel, and Benefiber, which cause the bulking of stools
  • Iron supplements, which can cause grayish staining of the intestinal walls and should not be taken a day before the procedure
  • Certain diabetic medications, which may require a dose reduction a day before the procedure and complete stoppage on the morning of the procedure

Dietary Changes (3 Days Prior)

One week or three days before the procedure, you will be asked to avoid high-fiber foods, including salads, green vegetables, brown rice, whole grain bread, brown pasta, beans, sunflower seeds, peanuts, cashews, and popcorn.

One day before the procedure, your healthcare provider will place you on a clear liquid diet. A liquid is considered clear if you can read through it. For example, apple juice would be considered acceptable, while orange juice or milk would not. Other acceptable liquids include lemon-lime soda, ginger ale, club soda, black coffee (no creamer or milk), clear sports drinks, mineral water, and water.

For nutrition, you can eat clear chicken, beef, or vegetable broth as well as yellow or green gelatin or popsicles. (Avoid red, purple, or orange-colored drinks, gelatins, or popsicles which may discolor intestinal tissues). Sweeteners like sugar and honey are also okay.

You would also need to avoid alcohol, including beer and wine, the day before your procedure.


Bowel Prep Timeline (1 Day Prior)

One day before your procedure, you would begin the actual bowel preparation, which involves laxatives and other medications you need to take on a prescribed schedule. The timelines and procedures can vary, but here is an example commonly used for a colonoscopy bowel prep:

  • Upon waking, eat or drink nothing but clear liquids.
  • At 12:00 noon, take the prescribed dose of laxative, like Dulcolax (bisacodyl), with a full glass of water.
  • At 5:00 p.m., you would need to mix a product called Gavilyte or Colyte (polyethylene glycol with sodium bicarbonate and salt) with a gallon of water. Some people add powdered flavored drinks like Kool-Aid or Crystal Light to make it more palatable. (Avoid red, orange, or purple flavors.)
  • At 6:00 p.m., drink 8 ounces of the Gavilyte/Colyte solution every 10 to 15 minutes until two-thirds is consumed. Save the rest in the refrigerator for the following morning.
  • At 9:00 p.m., chew two simethicone tablets. Simethicone is an antifoaming agent that helps decrease gas and bloating.
  • At 10:00 p.m, take two more simethicone tablets.

Depending on your sensitivity to the laxative, you may start experiencing clearance around midday. The major clean-out may occur with the consumption of the Gavilyte/Colyte solution, so be sure to stay close to the toilet. You may also need to get up in the middle of the night, so keep a nightlight on in the bathroom.

Although this timeline provides you with a relatively strong idea of what to expect, always refer to your healthcare provider's instructions above all else and without deviation.


On the Morning of Your Procedure

Upon waking, finish all of the remaining Gavilyte / Colyte solutions, drinking 8 ounces every 10 to 15 minutes. You should finish this no later this three hours before the procedure and neither drink nor eat anything after. If you need to take medications, do so with small sips of water first thing in the morning.

Some people like to lightly enema in the morning to ensure that there is no remaining residue in the bowel. If you did the bowel prep correctly, this is usually not necessary.

If you do decide to use an enema, do so lightly. Not only does overfilling the colon risk a messy accident on the way to the healthcare provider's office, but it can also cause inflammation and swelling that can compromise the delicate tissues of the rectum and colon.


Side Effects and Risks

Generally speaking, bowel prep is safe and poses little risk to your health. Discomfort is the most common complaint with people experiencing such symptoms as:

  • Abdominal cramping
  • Nausea or vomiting
  • An uncomfortable feeling of Fullness
  • Fecal Incontinence
  • Changes in taste
  • Sudden, cold sweating

In rare instances, the polyethylene glycol (PEG) in Gavilyte and Colyte can cause acute kidney failure and kidney damage. Most cases involve people with pre-existing kidney disease, but some cases have been noted in people with no known history of kidney dysfunction.

Other serious symptoms can occur if you lose more water in your bowel movements than you replenish. This can lead to severe dehydration, the symptoms of which include:

  • Decreased urine output
  • Dizziness or lightheadedness
  • Rapid heartbeat or palpitations
  • Rapid breathing
  • Fatigue and weakness
  • Shortness of breath
  • Confusion
  • Fainting

The best way to monitor your hydration status is to look at the color of your urine. Dark urine indicates that you need more fluid, while light yellow or colorless urine indicates adequate hydration. 

Drinking plenty of fluid will not only improve your overall hydration, but it may also reduce the impact of PEG on your kidneys.


Helpful Tips

There is no pretending that bowel prep is anything less than unpleasant, but there are things you can do to minimize the discomfort and reduce potential risks. Among them:

  • In addition to flavoring the Gavilyte/Colyte solution, keeping it cold in the refrigerator helps reduce some of the off-putting taste.
  • Purchase extra-soft, unscented toilet paper to minimize rectal inflammation and soreness. You can also use disposable baby wipes that are gentler on the skin.
  • After wiping, apply a barrier ointment, like those used for diaper rash, to keep the anal tissues soft and moisturized.
  • To avoid bathroom accidents, wear soft, loose-fitting clothes that can be easily removed, like pajama bottoms or boxer shorts.
  • If you are feeling lightheaded during your bowel prep, slow your intake of Gavilyte/Colyte. Also, try to bear down less when you are on the toilet as this can sometimes cause a feeling of lightheadedness. 
  • If you are diabetic, be sure that your healthcare provider is aware of this. Going without food for long stretches of time can cause problems, so be sure to have a plan in place in the event your blood sugar drops. Instead of just water, you may want to drink something like Sprite if your glucose reading is low.
  • If you followed the instructions but your bowel fluids are coming out dark or gritty, call the healthcare provider's office for further instructions. If you are not entirely clear by the morning, you may need to cancel if only to avoid compromising the procedure or risking infection.
6 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.
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  2. Memorial Sloan Kettering Cancer Center. How to Prepare for Your Colonoscopy Using MiraLAX®.

  3. Harvard Medical School. Preparing for a colonoscopy.

  4. Hilsden RJ. Seeking the ultimate bowel preparation for colonoscopy: is the end in sight?. Can J Gastroenterol. 2011;25(12):655-6. doi:10.1155/2011/628786

  5. U.S. National Library of Medicine. MedlinePlus. Dehydration.

  6. Fight Colorectal Cancer. Colonoscopy prep: 8 expert tips for the night before.

Additional Reading

By Jennifer Whitlock, RN, MSN, FN
Jennifer Whitlock, RN, MSN, FNP-C, is a board-certified family nurse practitioner. She has experience in primary care and hospital medicine.