You need to stop eating solid food one full day before your colonoscopy and stop drinking all liquids, including water, two hours before your scheduled procedure time. Those are the two key cutoffs, but the full prep timeline starts earlier and involves more than just not eating.
The Full Fasting Timeline
The prep unfolds in stages. Starting one full day before your procedure, you switch entirely to a clear liquid diet. If your colonoscopy is Thursday morning, your last solid meal is Tuesday dinner. All of Wednesday, from the moment you wake up, is liquids only.
Then, on the morning of your procedure, you stop drinking clear liquids two hours before your scheduled arrival time. This final two-hour window exists because of sedation safety. The American Society of Anesthesiologists sets this rule for any procedure involving sedation or anesthesia: clear liquids are safe up to two hours before, but nothing after that. An empty stomach prevents the risk of inhaling liquid into your lungs while you’re sedated.
Your prep solution (the large-volume drink that clears your bowels) is typically split into two doses. You’ll drink the first dose the evening before and the second dose early the morning of your procedure, finishing it well before that two-hour cutoff. Your doctor’s office will give you exact timing based on your appointment.
What You Can and Can’t Drink
A clear liquid diet means anything you can see through. That includes plain water, black coffee or tea (no cream or milk), clear broths like bouillon or consommé, apple juice without pulp, cranberry juice, ginger ale, Sprite, sports drinks, plain gelatin, and popsicles without fruit bits or yogurt.
What’s off the list: anything with pulp (like prune juice or orange juice), milk, yogurt, smoothies, and alcohol. Your doctor will likely also tell you to avoid red or purple liquids, since the coloring can look like blood during the procedure and make it harder for the gastroenterologist to see clearly.
Why the Prep Matters More Than You Think
It’s tempting to cut corners on prep, but the quality of your bowel preparation directly affects whether the procedure can find what it’s looking for. A large study of over 335,000 colonoscopies published in the American Journal of Gastroenterology found that people with inadequate bowel prep were less than half as likely to have precancerous polyps detected compared to those with excellent prep. The odds of detection dropped to 0.44, meaning doctors missed more than half the growths they would have otherwise caught.
Even slightly suboptimal prep made a difference. Flat, harder-to-spot precancerous lesions were about 16 to 17 percent less likely to be detected when prep quality dropped from excellent to merely “good.” These are the growths most likely to be missed during a colonoscopy, and poor prep makes them nearly invisible. If your prep is truly inadequate, you may need to come back and do the entire process over again.
Medications During the Fast
Most regular medications, including blood pressure pills, heart medications, antibiotics, and diuretics, can be taken on the morning of your procedure with a small sip of water. That small sip doesn’t violate the fasting rules.
Diabetes medications are the major exception and require more planning. If you take oral diabetes medications like metformin, you’ll typically need to stop them 48 hours before the procedure. Weekly injectable medications like semaglutide should be held if your next dose falls within 48 hours of the colonoscopy. If you use long-acting insulin, the general approach is to take half your usual dose the day before and skip the morning dose entirely. These adjustments vary by medication and by patient, so your doctor’s office should give you a specific plan for your situation.
What the Day Before Actually Looks Like
The day before your colonoscopy is the hardest part. You’ll be hungry, you’ll be drinking a large volume of prep solution that most people find unpleasant, and you’ll be spending a lot of time in the bathroom. A few things make it more manageable.
Stock up on a variety of clear liquids so you’re not stuck drinking only water and broth for 24 hours. Chicken broth, clear juices, and gelatin give you some calories and flavor variety. Drinking the prep solution cold, through a straw, and with a chaser of ginger ale helps with the taste. Stay near a bathroom from the evening onward, because the prep works thoroughly and quickly.
Plan to finish your second dose of prep solution early enough that you’re done drinking at least two hours before you need to arrive. If your procedure is at 9 a.m. and your arrival time is 8 a.m., you’d want to finish all liquids by 6 a.m. at the latest. Your prep instructions will spell out the exact timing.
After the Procedure
Once the colonoscopy is done, which typically takes 30 to 60 minutes, you can eat again as soon as you feel ready. Most people start with something light and bland since your digestive system has been completely emptied. If you take diabetes medications that were paused, you’ll generally resume them 24 hours after the procedure once you’re eating regular meals again. Insulin can restart once you’re tolerating food normally.
The sedation takes a few hours to fully wear off, so you’ll need someone to drive you home. Most people feel back to normal by the next day.