Miralax contains a single active ingredient: polyethylene glycol 3350, delivered as 17 grams per dose. There are no inactive ingredients, fillers, flavors, sugar, gluten, or preservatives. It’s one of the simplest over-the-counter medications you’ll find, essentially just a white, odorless, tasteless powder that you dissolve in liquid before drinking.
What Polyethylene Glycol 3350 Actually Is
Polyethylene glycol 3350 (often shortened to PEG 3350) is a synthetic compound that dissolves easily in water. The “3350” refers to its molecular weight, which determines how the substance behaves in your body. Despite the industrial-sounding name, it’s widely used in pharmaceuticals and personal care products. In Miralax, it functions as an osmotic laxative, meaning it works by pulling and holding water in the colon rather than stimulating muscle contractions.
Your body barely absorbs it. FDA pharmacology data shows that only 0.1 to 0.2% of an oral dose enters the bloodstream. The vast majority, roughly 60 to 70%, passes straight through and exits in stool. A small portion (10 to 20%) is eliminated through urine. In practical terms, the powder travels through your digestive tract mostly untouched, doing its work along the way.
How It Works in Your Body
PEG 3350 forms hydrogen bonds with water molecules. This means it grabs onto water in your intestines and prevents your colon from reabsorbing it the way it normally would. The result is more water staying in your stool, which softens it and increases its bulk. That extra volume also raises osmotic pressure in the colon, which triggers more frequent bowel movements.
This is fundamentally different from stimulant laxatives (like bisacodyl or senna), which work by forcing the muscles of your intestinal wall to contract. Stimulant laxatives can cause cramping and, with long-term use, have been linked to structural changes in the colon, including loss of the natural folds that help move stool along. Osmotic laxatives like Miralax avoid this because they don’t act on muscles or nerves at all.
No Hidden Ingredients
One of the most common reasons people search “what is in Miralax” is concern about additives. The FDA label for Miralax lists no inactive ingredients. There’s no sugar, no artificial sweetener, no coloring, no gluten, and no preservatives. The powder itself is flavorless, which is why the instructions tell you to mix it into any beverage of your choice. Each dose is measured by filling the cap to the marked line (17 grams) or by using a pre-measured packet.
Standard Dosage and Preparation
A single adult dose is 17 grams of powder mixed into 4 to 8 ounces of any liquid, hot or cold. Miralax comes in bottles (119-gram or 238-gram sizes) or in boxes of pre-measured 17-gram packets. You take one dose per day. The product typically produces a bowel movement within one to three days, not immediately like a stimulant laxative.
The 238-gram bottle is also used for colonoscopy preparation, but at much higher doses and mixed with 64 ounces of clear liquid, taken over several hours. This is a completely different protocol from regular constipation use and is done under a doctor’s direction.
Side Effects Are Minimal
In a randomized, placebo-controlled clinical trial, side effect rates for PEG 3350 were nearly identical to placebo. About 48% of people taking the active drug reported some kind of side effect, compared to 56% on placebo. The most commonly reported gastrointestinal issues were abdominal pain (12.9% on PEG 3350 vs. 17.6% on placebo), nausea (12.9% vs. 5.9%), and diarrhea (6.5% vs. 2.9%). Bloating and gas were actually more common in the placebo group. In short, most of the symptoms people associate with Miralax occur at similar rates even when people aren’t taking it.
Because PEG 3350 is barely absorbed, it doesn’t interact meaningfully with other medications or affect your organs. This is one reason it has become a first-line treatment for constipation across age groups.
Use in Children
Miralax is FDA-approved for adults, but it’s widely used in pediatric settings. Both the North American and European societies for pediatric gastroenterology recommend PEG 3350 as the first-line treatment for childhood constipation, including for clearing fecal impaction. Studies show it’s more effective than lactulose, milk of magnesia, and mineral oil.
For children, the dose is weight-based, typically starting at 0.4 grams per kilogram of body weight per day for ongoing constipation. For impaction, higher doses of 1 to 1.5 grams per kilogram per day may be used for three to six days. Pediatric guidelines recommend continuing maintenance treatment for at least two months after symptoms resolve and tapering off gradually rather than stopping abruptly. For children in the process of toilet training, the recommendation is to continue treatment until training is complete.
How It Compares to Other Laxatives
Laxatives fall into a few broad categories, and knowing where Miralax fits helps explain why it’s so commonly recommended.
- Osmotic laxatives (Miralax, lactulose, magnesium citrate) hold water in the colon to soften stool. They work gently and carry low risk of dependency.
- Stimulant laxatives (bisacodyl, senna) trigger muscle contractions in the intestinal wall. They work faster but can cause cramping, and long-term use is associated with structural changes to the colon.
- Bulk-forming laxatives (psyllium, methylcellulose) add fiber to increase stool volume. They require significant water intake and can cause bloating.
- Stool softeners (docusate) add moisture to stool but are generally considered less effective than osmotic options for moderate to severe constipation.
Miralax’s advantage is its simplicity. One ingredient, virtually no absorption, no taste, and a side effect profile that’s statistically no worse than taking nothing at all. That combination is why it’s become the default recommendation for occasional constipation in both adults and children.