Polyethylene glycol 3350 (sold as MiraLAX and various generics) is mixed into a clear liquid and drunk in two separate doses to empty your bowels before a colonoscopy. The standard prep uses one 238-gram bottle of the powder dissolved in 64 ounces of a clear sports drink or similar liquid, split evenly between the evening before and the morning of your procedure.
What You Need Before You Start
Pick up one 238-gram (8.3-ounce) bottle of polyethylene glycol 3350 from the laxative aisle. You’ll also need 64 ounces of a noncarbonated clear liquid. Sports drinks like Gatorade or Propel work well because they replace electrolytes you’ll lose during the prep. Crystal Light and similar clear beverages also work.
Avoid any liquid that is red, orange, or purple. These dyes coat the lining of the colon and make it harder for the doctor to see during the exam. Stick with yellow, green, or clear options. Milk, dairy products, and alcohol are also off limits during the entire prep period.
Some doctors also prescribe a stimulant laxative tablet to take alongside the PEG 3350. If your instructions include these tablets, follow the specific timing your clinic provides. A common protocol calls for four tablets taken at 1:00 p.m. on each of the two days before the procedure.
The Diet Timeline
Most prep protocols have you switch to a low-residue diet (white bread, eggs, lean chicken, no raw vegetables or whole grains) starting a few days before the procedure. You’ll typically stop that diet by early afternoon the day before your colonoscopy, usually around 1:00 p.m., and move to clear liquids only. Clear liquids include broth, plain gelatin (again, no red, orange, or purple), clear juice like apple or white grape, water, tea, and black coffee.
Mixing and Drinking the Prep
The day before your colonoscopy, pour the entire 238-gram bottle of PEG 3350 into a pitcher with 64 ounces of your chosen clear liquid. Stir until the powder fully dissolves. It usually dissolves easily and doesn’t change the taste of the liquid much, which is one reason this prep is popular compared to older, saltier alternatives. Refrigerating the mixture makes it easier to drink.
The prep is split into two rounds of 32 ounces each:
- Evening dose: Starting around 6:00 p.m. the night before, drink 32 ounces total. Pour yourself an 8-ounce glass every 15 minutes until you’ve finished the first half.
- Morning dose: Six hours before your scheduled colonoscopy time, drink the remaining 32 ounces at the same pace: one 8-ounce glass every 15 minutes.
This split-dose approach is the current standard because it produces a cleaner colon than drinking everything in one sitting the night before. Research shows the best bowel cleansing happens when you finish your last dose about three to five hours before the procedure, with anything up to seven hours still producing acceptable results.
What to Expect as the Prep Works
Bowel movements typically start within one to two hours after your first dose. Your stool will change in a predictable sequence: it begins as normal brown and solid, then softens and lightens to a tan color, then becomes watery and orange. The goal is a light yellow liquid with no dark particles. Once your output looks like pale lemonade, your prep is complete. If you still see brown flecks or cloudy material after finishing all the solution, the colon may not be fully clean, but continue with your appointment as scheduled and let your doctor know.
Expect to spend a lot of time in the bathroom, especially in the first two to three hours after each dose. Keeping your phone charger nearby and using a barrier cream or petroleum jelly around the anus before you start can make the experience more comfortable. The frequent wiping is often the most unpleasant part.
Dealing With Nausea and Vomiting
Nausea, bloating, abdominal cramping, and sometimes vomiting are common side effects. If you feel nauseous while drinking the prep, pause for 30 minutes, rinse your mouth with water, and then resume. Drinking the solution cold and using a straw can also help. If you vomit, don’t panic. Drink as much of the remaining prep as you can tolerate and still report for your colonoscopy at the scheduled time. Let your doctor’s office know ahead of the procedure that you weren’t able to finish the full volume.
Thirst and dizziness can happen because you’re losing a large amount of fluid. Drinking extra clear liquids between prep doses helps. Just avoid anything with color restrictions or dairy.
Who Should Use Caution
PEG 3350 prep is not safe for anyone with a known or suspected bowel obstruction, a perforated bowel, appendicitis, or active inflammatory bowel disease. People with kidney problems or existing electrolyte imbalances need extra monitoring, and their doctors will often choose a modified prep or adjust the protocol.
Adjustments for Diabetes
If you take diabetes medications, the combination of fasting and heavy fluid loss requires careful planning. The general principles vary by medication type. Metformin should be stopped once you begin the clear liquid diet and restarted after you’re eating regular meals again. Sulfonylureas (pills that stimulate insulin release) should be stopped the full day before the procedure. SGLT-2 inhibitors, a newer class of diabetes pills, need to be stopped three full days before the colonoscopy because they increase the risk of a dangerous acid buildup when combined with dehydration and fasting.
If you take injectable diabetes medications like weekly GLP-1 shots, hold the dose starting two days before the procedure. For insulin users, the adjustments depend on the type: rapid-acting insulin is typically cut to 50% of the usual fixed dose during the clear liquid phase, while long-acting basal insulin is reduced to 50-80% of the normal dose starting the day before. Your prescribing doctor should give you a specific plan for your regimen, since getting this wrong can cause dangerous blood sugar swings during a period when you’re unable to eat normally.
Quick Reference Timeline
- 2-3 days before: Begin a low-residue diet. Take any prescribed stimulant laxative tablets if instructed.
- Day before, early afternoon: Switch to clear liquids only. Stop eating all solid food.
- Day before, 6:00 p.m.: Begin drinking the first 32 ounces of the PEG 3350 mixture, one glass every 15 minutes.
- Day of, 6 hours before procedure: Drink the second 32 ounces at the same pace.
- 3-5 hours before procedure: All prep liquid should be finished. Stop drinking all liquids at whatever cutoff time your clinic specifies (commonly two hours before).
Following this split-dose schedule closely is the single most important thing you can do to ensure a clean colon and a successful exam. A poor prep can mean the doctor misses polyps or, worse, that you need to repeat the entire process for a second colonoscopy.