Colonoscopy Prep: What to Eat, What to Drink and When to Start
Updated: 6 days ago
Written and medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: October 2026 · How we review
Quick Answer: What is colonoscopy prep?
Colonoscopy prep is the process of emptying and cleaning your colon before a colonoscopy so your doctor can clearly see its lining and spot polyps. It usually means a low-fiber diet two to three days before, clear liquids only the day before, and a laxative drink split between the day before and four to six hours before the procedure. [1]
What the research says
Skipping steps is a leading cause of a poor prep. A meta-analysis of 154 studies with 358,257 people found that not using a split-dose prep, not finishing the prep, and not following diet instructions were all linked to an inadequate bowel prep. Constipation, diabetes and opioid use were also risk factors. (Beran et al., Am J Gastroenterol, 2024)
Some prep combinations clean better than others. A network meta-analysis of 22 randomized trials (7,179 patients, 14 preps) found 2 L PEG with simethicone was 25% more likely to give a successful clean-out than a standard 2 L PEG-ascorbate prep, and it also improved polyp (adenoma) detection. (Maida et al., Dig Liver Dis, 2025)
Expert guidelines favor split dosing. The US Multi-Society Task Force recommends a split-dose regimen, says a 2 L prep may be enough, and sets a target of more than 90% of colonoscopies having an adequate prep. (Jacobson et al., Gastroenterology, 2025)
Key takeaways
A clean colon lets your doctor see polyps that poop could otherwise hide.
Switch to a low-fiber diet two to three days before, then clear liquids only the day before.
Avoid red, purple and orange drinks and gelatin — they can look like blood.
Most kits use split dosing: one dose the day before, one dose about four to six hours before.
Your prep is done when your stool is a clear yellow liquid you can see through.
Call your provider if nothing happens after five to six hours, or if you have severe pain or vomiting.
What is colonoscopy prep?
Colonoscopy prep is the process of emptying and cleaning out your colon before a colonoscopy. It's also called bowel preparation or bowel prep.
It usually has two parts: a special diet and a laxative solution you drink, often called a prep drink. [1]
Most prep kits combine two things:
Part of the prep | What it does |
Osmotic laxative | Pulls water into your colon. This softens stool, adds volume and stretches the colon wall, which triggers the muscles to push stool out. |
Clear liquids with electrolytes | Prevent dehydration while you empty your bowels. Clear sports drinks are a common choice. |
Learn more about how these medicines work in our guide to laxatives.
Why does colonoscopy prep matter?
Colonoscopy prep matters because a clean colon is the only way your doctor can clearly see its lining. Signs of colon cancer, such as polyps, often stick to the inside walls of the colon. [1]
Leftover stool can hide these areas. That makes problems harder to spot.
If your colon isn't clear on the day of the procedure, your colonoscopy may be canceled or the results may not be accurate. You may have to repeat the prep and reschedule. [1]
What types of colonoscopy prep kits are there?
Colonoscopy prep kits fall into two main groups based on the type of laxative they use. Your provider chooses one based on your health history and medicines. [1]
Polymer-based (PEG) | Saline-based (sodium phosphate) | |
Main ingredient | Polyethylene glycol 3350 | Sodium phosphate or related salts |
Form | Powder mixed with large amounts of water or sports drink | Ready-mixed liquid in bottles, or tablets taken with lots of clear fluids |
Examples | GoLYTELY, NuLYTELY, TriLyte, Colyte, MiraLAX, HalfLytely, GaviLyte, MoviPrep | Clenpiq, Suprep, Prepopik, OsmoPrep, SUTAB |
Time to start working | About 1 to 3 hours | About 3 to 6 hours |
Every kit has pros and cons. Tell your provider about all your medical conditions and current medicines so they can pick the right one. Sodium phosphate tablets (OsmoPrep) carry an FDA boxed warning for rare but serious kidney damage, so they are usually avoided in people with kidney disease, heart failure, dehydration or certain blood pressure medicines. For a closer look at the kit itself, see our bowel prep kit guide.
When do you start colonoscopy prep?
Most people start colonoscopy prep the day before the procedure. The exact timing depends on your kit and the time of your appointment. [1]
Most kits use split dosing:
Dose 1: the day before your colonoscopy (usually afternoon or evening)
Dose 2: about four to six hours before your procedure
Read your instructions several days ahead. This gives you time to buy clear liquids and sports drinks.
What can you eat before a colonoscopy?
Two to three days before a colonoscopy, you switch to a low-fiber diet. High-fiber foods tend to stick to the colon wall. [1]
Stop eating these foods during that time:
Stop 2–3 days before | Why |
Seeds | Can stick to the colon lining |
Nuts | Can stick to the colon lining |
Beans | High in fiber |
Raw vegetables | High in fiber |
Popcorn | Leaves residue in the colon |
Your provider may also adjust some of your medicines before the procedure. Share your full medicine list when you book. [1]
What can you drink the day before a colonoscopy?
The day before a colonoscopy, you follow a clear liquid diet and eat no solid food at all. [1]
OK to drink | Avoid |
Water | Alcohol |
Clear sodas, such as ginger ale or lemon-lime soda | Milk or non-dairy creamers |
Tea and black coffee (no milk or cream) | Broth with noodles or vegetables |
Chicken or vegetable broth or bouillon | Juices with pulp |
Sports drinks (not red, purple or orange) | Dark-colored drinks, except black coffee |
Gelatin (not red, purple or orange) | Any solid food |
Pulp-free apple juice or clear grape juice |
Red and purple colors matter because they can look like blood inside your colon. [1]
Drinking enough clear fluids also helps you avoid dehydration while the prep works.
When should you stop drinking before a colonoscopy?
You usually stop drinking clear liquids about two to four hours before your colonoscopy. Your provider's instructions should list the exact time. [1]
Follow these instructions exactly to avoid having your colonoscopy canceled.
How long does colonoscopy prep take to work?
Colonoscopy prep takes about one to three hours to start with a PEG-based formula and about three to six hours with a sodium phosphate formula. [1]
Contact your provider if you're constipated and still not passing stool several hours after starting. You may need extra steps to get things moving.
How do you know your colonoscopy prep is done?
Your colonoscopy prep is done when your stool is a clear yellow liquid that you can see through. [1]
If you started on time, you shouldn't be having many bowel movements by the time you arrive. Some people wear pads or protective underwear on the way, just in case.
When should you call your doctor during colonoscopy prep?
Call your healthcare provider during colonoscopy prep if any of these happen: [1]
You forgot to stop eating or to start your prep on time
You get sick and need to reschedule
You're not passing stool after five to six hours
You have severe belly pain
You're vomiting or can't keep fluids down
You don't think you can finish the prep
Colonoscopy prep at a glance
When | What to do |
Several days before | Read your kit instructions; buy clear liquids and sports drinks |
2–3 days before | Low-fiber diet; no seeds, nuts, beans, raw vegetables or popcorn |
Day before | Clear liquids only; first prep dose as instructed |
4–6 hours before | Second prep dose (split dosing) |
2–4 hours before | Stop all clear liquids (or as instructed) |
Arrival | Stool should be clear yellow liquid |
Conclusion
Colonoscopy prep is just as important as the procedure itself. A clean colon helps your doctor find polyps early, and a poor prep can mean a canceled test or a repeat. [1]
Most of the work comes down to three steps: eat low-fiber foods for a few days, switch to clear liquids the day before, and take your prep drink exactly on schedule. Knowing what to expect makes the whole process much easier to manage.
Have a colonoscopy coming up? Save this guide, share it with anyone preparing for theirs, and read our guides to colon polyps and polyp removal (polypectomy) so you know what may happen during the test. Questions about your own kit? Ask your provider — they'll tailor the plan to you.
FAQ
Can I drink coffee before a colonoscopy?
Yes. Black coffee is allowed on the clear liquid diet the day before a colonoscopy. Don't add milk, cream or non-dairy creamer. [1]
Can I eat solid food the day before a colonoscopy?
No. The day before a colonoscopy, you follow a clear liquid diet only, with no solid food. [1]
Why can't I have red or purple drinks before a colonoscopy?
Red and purple drinks and gelatin can look like blood inside your colon. Orange-colored drinks and gelatin are also best avoided. [1]
How long does colonoscopy prep take to start working?
PEG-based preps usually start in about one to three hours. Sodium phosphate preps can take about three to six hours. [1]
What if I'm still passing stool before my colonoscopy?
If you started your prep on time, you shouldn't have many bowel movements by arrival. The prep is complete when stool is clear yellow liquid. Pads or protective underwear can help on the trip. [1]
Do I need to stop any medicines before a colonoscopy?
Your provider may adjust some medicines before the procedure. Share your full medicine list when you schedule your colonoscopy so they can tell you what to change. [1]
What should I do if I throw up during colonoscopy prep?
Stop drinking for a short time, then try again more slowly. If you keep vomiting or cannot keep fluids down, call your provider, because your prep or procedure may need to change.
How can I make the prep drink easier to finish?
Chilling the drink, sipping it through a straw and taking a sip of a clear drink in between can help. Do not add anything to the prep unless your instructions say it is OK.
How can I prevent a sore bottom during prep?
Use soft toilet paper or flushable wipes, and pat instead of rubbing. A barrier cream, such as petroleum jelly, can protect your skin.
Why is the prep often split into two doses?
Taking half the night before and half the morning of the test usually cleans the colon better. A cleaner colon makes it easier for your doctor to find polyps.
Related Reading
Colonoscopy: Purpose, Prep, Procedure, Results and Risks explains what happens during a colonoscopy and what results mean.
Preparing for Your Colonoscopy: A Guide to the Bowel Prep Kit compares bowel prep kits and how to take them correctly.
Clear Liquid Diet: What It Is, Foods to Eat & Avoid, and How Long It Lasts lists which drinks and foods are allowed on a clear liquid diet.
Colon Polyps: Complete Guide to Symptoms, Causes, and Treatment explains colon polyps, their cancer risk and how they are removed.
References
Cleveland Clinic. How To Prepare for a Colonoscopy: Foods Allowed & Bowel Prep. Cleveland Clinic. Last updated May 15, 2026. https://my.clevelandclinic.org/health/treatments/22657-colonoscopy-bowel-preparation
Beran A, Aboursheid T, Ali AH, et al. Risk Factors for Inadequate Bowel Preparation in Colonoscopy: A Comprehensive Systematic Review and Meta-Analysis. Am J Gastroenterol. 2024;119(12):2389-2397. PMID 39225554. https://doi.org/10.14309/ajg.0000000000003073
Maida M, Marasco G, Fuccio L, et al. Comparative efficacy of different bowel preparations for colonoscopy: A network meta-analysis. Dig Liver Dis. 2025;57(3):688-696. PMID 39676010. https://doi.org/10.1016/j.dld.2024.11.019
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. Gastroenterology. 2025;168(4):798-829. PMID 40047732. https://doi.org/10.1053/j.gastro.2025.02.002
Disclaimer
This article is for general education only and is not a substitute for professional medical advice. Always follow the specific bowel prep instructions from your own healthcare provider, and contact them with any questions about your colonoscopy.
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