Polyethylene glycol 3350 is an osmotic laxative used to treat occasional constipation. It’s the active ingredient in MiraLAX and several store-brand equivalents, and it’s available over the counter in powder form. In higher-volume prescription formulations mixed with electrolytes, it’s also used to clean out the bowel before a colonoscopy.
The “3350” refers to its molecular weight, which distinguishes it from other polyethylene glycol compounds used in industrial or pharmaceutical products. At this specific size, the molecule passes through your digestive tract without being absorbed into the bloodstream, which is what makes it useful as a laxative.
How It Works
PEG 3350 belongs to a class of drugs called osmotic laxatives. When you drink it dissolved in liquid, it travels to your colon and pulls water into the stool through osmosis. The extra water makes stool softer, bulkier, and easier to pass. Unlike stimulant laxatives that force the muscles of your intestine to contract, PEG 3350 simply changes the water content of what’s already moving through your system. This makes it gentler and less likely to cause cramping.
What It’s Used For
The over-the-counter version treats occasional constipation in adults. It’s typically recommended for short-term use (up to seven days), though some doctors prescribe it for longer periods in people with chronic constipation.
Prescription formulations combine PEG 3350 with electrolytes (sodium, potassium, and bicarbonate) and are used for bowel cleansing before a colonoscopy. These prep solutions require drinking a much larger volume, often 3 to 4 liters total, to completely flush the colon so doctors can see the lining clearly during the procedure.
How to Take the OTC Version
The standard dose is 17 grams of powder per day, which works out to about one heaping tablespoon. You stir it into 4 to 8 ounces of water, juice, soda, coffee, or tea until it dissolves. The powder is tasteless and odorless, which is one reason doctors commonly recommend it over other laxatives.
PEG 3350 doesn’t work immediately. Most people have a bowel movement within one to three days of their first dose. It’s not the right choice if you need fast relief; it works gradually by accumulating water in the colon over time.
Common Side Effects
The OTC constipation dose causes fewer problems than the high-volume colonoscopy prep, but mild digestive symptoms are still possible. These include bloating, gas, nausea, and loose stools. Most people tolerate it well at the standard 17-gram dose.
The colonoscopy prep version, taken in much larger quantities, causes significantly more discomfort. In clinical data, roughly 60% of patients reported bloating, about half experienced anal discomfort, nearly half had nausea, and around 39% had abdominal pain. Vomiting occurred in about 13% of patients. These effects are temporary and resolve once the prep is finished.
Who Should Not Take It
PEG 3350 should not be used by anyone with a known or suspected bowel obstruction. Symptoms of a bowel obstruction include severe abdominal pain, distension, nausea, and vomiting. If you have these symptoms, the cause needs to be identified before starting any laxative. People with a known allergy to polyethylene glycol should also avoid it.
Because PEG 3350 can affect how quickly other medications move through your digestive tract, timing matters if you take other drugs by mouth. The general guidance for the prescription prep version is to avoid taking other medications within one hour of a dose, and certain drugs (including heart medications like digoxin, iron supplements, and some antibiotics) should be taken at least two hours before or six hours after. For the OTC version, spacing your other medications by an hour or two is a reasonable precaution.
Use in Children
Pediatricians sometimes recommend PEG 3350 off-label for constipation in children, and the prescription bowel prep version is FDA-approved for children as young as 6 months. However, the use of PEG 3350 in children has drawn some scrutiny. In 2009, the FDA’s Drug Safety Oversight Board reviewed reports of neuropsychiatric and metabolic events in children taking PEG 3350 laxatives. The reported events included seizures, tremors, tics, anxiety, aggression, and mood changes. The board acknowledged the reports but concluded at that time that no regulatory action was needed.
These reports came primarily from the FDA’s adverse event reporting system, which collects voluntary reports and does not establish that a drug caused a particular symptom. Still, the reports prompted ongoing discussion. If your child is taking PEG 3350 and you notice behavioral or neurological changes, that’s worth raising with their pediatrician.
What Colonoscopy Prep Looks Like
If you’ve been prescribed a PEG 3350 prep for a colonoscopy, the experience is more involved than the daily constipation dose. A common protocol from Cleveland Clinic uses a split-dose approach: you drink 12 glasses (about 3 liters) of the solution the evening before the procedure, one glass every 10 minutes, then finish the remaining 4 glasses (1 liter) four hours before your appointment the next morning. The goal is to keep drinking steadily rather than gulping large amounts at once, which helps reduce nausea.
By the end of the prep, your bowel movements should be clear or light yellow liquid, which signals that the colon is clean enough for the procedure. Most people find the volume of liquid the hardest part. Chilling the solution and drinking it through a straw can make it slightly more tolerable.