Colonoscopy Prep, Hour by Hour: What the Day Before Really Looks Like

For most people, the hardest part of a colonoscopy is the day before it. Colonoscopy prep means a day of clear liquids, a laxative drink, and a lot of time in the bathroom. Prep plans differ between practices and between patients, so the written instructions from your own care team always come first. GI Associates patients can find our prep sheets on the prep instructions page.

A week or so before: medicines to ask about

Read your instructions early. Some steps start days ahead, and you may need to call your prescribing doctor.

Tell your care team about every medicine and supplement you take. Ask specifically about:

• Blood thinners, including aspirin and other anti-clotting medicines. Some people are told to pause them, and many are told to keep taking them. Never stop one on your own, especially if you have a heart condition.
• Diabetes medicines, including insulin. A day without solid food can make your blood sugar drop, so your dose or timing may need to change. Ask how often to check your sugar on the liquid day, and keep a quick source of sugar on hand.
• GLP-1 medicines, the shots or pills used for diabetes or weight loss. They slow how fast the stomach empties, so your team may give you special instructions, such as a longer liquid diet. Don’t skip a dose unless your team tells you to.
• Iron pills or liquids. Iron can turn stool dark and make the colon harder to see, so ask whether and when to pause it.

Also mention kidney or heart disease, an irregular heartbeat or past seizures, which can change which prep is safest.

Line up your driver now too, since you’ll get sedation and can’t drive yourself home.

Two to five days before: a lighter diet

Many plans call for a few days of low-fiber eating, which means holding off on nuts, seeds, popcorn, corn, whole grains, raw vegetables and fiber supplements. Newer national guidance allows a shorter diet change for many people at low risk of a poor prep, while people with constipation, diabetes or a past poor prep may get a longer plan. Your sheet says which applies.

The day before: clear liquids and the first dose

Morning. Depending on your plan, you switch to clear liquids first thing, after a light breakfast, or later in the day after low-fiber meals. Clear means you can see through it: water, broth, apple or white grape juice, plain tea or coffee without milk or creamer, light-colored sports drinks and gelatin. Skip anything red or purple, and skip alcohol. Drink steadily all day, because you’ll be losing a lot of fluid tonight.

Late afternoon or early evening. This is when the first dose often starts, though your sheet sets the exact time. Depending on the product, you drink a set amount of liquid over one to a few hours, or take tablets with plenty of water. Plan to be home.

The first few hours. Bowel movements usually begin within a few hours. You may pass some soft stool at first, and it becomes watery as the evening goes on. Expect urgency, cramping and bloating; some people feel chilled or a little nauseated.

Later that evening. As the colon empties, what comes out should get thinner and lighter. You’re aiming for liquid that is clear or light yellow, like urine, with no brown stool and no cloudiness. Many people don’t reach that point until after the second dose.

Overnight: the split dose and sleep

A split-dose prep puts part of it the evening before and the rest later. The U.S. Multi-Society Task Force on Colorectal Cancer recommends it, with the second dose usually started 4 to 6 hours before the procedure. For a morning exam, that can mean an alarm in the early hours. Some plans, including ours for early arrivals, set the second dose late the evening before, so go by your sheet.

Sleep will be broken, so go to bed once the evening trips slow down.

The morning of: still going?

If you’re still having watery bowel movements when it’s time to leave, that is common. Look at the color. Clear or yellow liquid means the prep is working. If you’re still passing brown liquid or solid stool after finishing all of the prep, call the office before you leave so the team can advise you.

Stop drinking at the time on your sheet. Many plans ask for nothing at all by mouth, not even water, gum or hard candy, for at least 2 hours before your procedure or arrival time, and some set a longer window. This protects your airway during sedation. Your driver takes you there and back. At GI Associates, that adult needs to stay on site until you’re discharged.

Tips that make it easier

• Chill the prep if your instructions allow it. Cold liquid is easier to get down, though it can leave you chilly.
• A straw, or sips of a clear drink you like between glasses, can help.
• If you feel queasy, slow down and take a short break rather than stopping.
• Use soft, moist wipes instead of dry tissue.
• If the skin gets sore, use the cream your prep sheet suggests, or ask your care team what to use.
• Set phone reminders for each dose and for your stop-drinking time.

When to call, and when to get urgent care

Call your care team if you can’t finish the prep, if you vomit, or if you have questions about any step. Don’t simply stop. A poor prep can hide polyps, and the exam may need to be repeated.

Get help right away if during the prep you have severe belly pain or swelling, repeated vomiting, fainting or dizziness that doesn’t pass, very little urine, a racing or irregular heartbeat, rectal bleeding, a seizure, trouble breathing, or low blood sugar that doesn’t improve after a sugary drink. Call your care team, and call 911 or go to the emergency room if symptoms are severe or you can’t reach anyone.

Once the exam is done, our colonoscopy recovery tips cover the rest of the day.

When to see a gastroenterologist

Screening for colorectal cancer is generally recommended starting at age 45 for people at average risk. You may need a colonoscopy at any age if you have rectal bleeding, a lasting change in bowel habits, iron-deficiency anemia, a positive stool test, or a family history of colon cancer or polyps.

GI Associates cares for patients in Wausau, Stevens Point, Wisconsin Rapids and the surrounding area. Every colonoscopy patient gets written prep instructions, and you can call the office with questions. Schedule a colonoscopy: request an appointment or call GI Associates at 715-847-2558.

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This article is for general education and is not a substitute for medical advice. Always follow the prep instructions from your own care team, and talk with your doctor before changing or stopping any medicine.

Sources

1. Jacobson BC, Anderson JC, Burke CA, et al. Optimizing Bowel Preparation Quality for Colonoscopy: Consensus Recommendations by the US Multi-Society Task Force on Colorectal Cancer. American Journal of Gastroenterology. 2025;120(4):738-764. https://doi.org/10.14309/ajg.0000000000003287
2. National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). Colonoscopy. https://www.niddk.nih.gov/health-information/diagnostic-tests/colonoscopy
3. MedlinePlus (U.S. National Library of Medicine). Polyethylene glycol-electrolyte solution (PEG-ES). Revised June 20, 2024. https://medlineplus.gov/druginfo/meds/a601097.html
4. American Cancer Society. How to Prep for a Colonoscopy: Diet Tips, When to Start, and More. March 17, 2025. https://www.cancer.org/cancer/latest-news/how-to-prep-for-a-colonoscopy.html
5. Kindel TL, Wang AY, Wadhwa A, et al. Multisociety Clinical Practice Guidance for the Safe Use of Glucagon-like Peptide-1 Receptor Agonists in the Perioperative Period. Clinical Gastroenterology and Hepatology. 2025;23(12):2083-2085. https://doi.org/10.1016/j.cgh.2024.10.003