MiraLAX is a brand name for polyethylene glycol 3350. The active ingredient listed on every MiraLAX label is polyethylene glycol 3350, 17 grams per dose. There is no other active ingredient, and no difference in the chemical compound between MiraLAX and store-brand versions of polyethylene glycol 3350.
Brand Name vs. Active Ingredient
MiraLAX is to polyethylene glycol 3350 what Tylenol is to acetaminophen. One is the brand name, the other is the drug itself. The FDA’s Center for Drug Evaluation and Research has formally determined there is “no meaningful difference” between MiraLAX and generic polyethylene glycol 3350 products. They share the same active ingredient, the same dosage form (powder dissolved in liquid), the same 17-gram dose mixed into 4 to 8 ounces of liquid, the same oral route, the same indication for occasional constipation, and the same approved patient population of adults 17 and older.
Generic versions sold under store labels at pharmacies and supermarkets contain the identical compound. If you see “polyethylene glycol 3350” on the label of a cheaper product, it is the same drug you’d get in a MiraLAX box.
How It Works
Polyethylene glycol 3350 is an osmotic laxative. It works by pulling water into the stool as it moves through your intestines, making the stool softer and easier to pass. Unlike stimulant laxatives, which trigger muscle contractions in your colon, PEG 3350 doesn’t force your bowels to move. It simply changes the water content of what’s already there.
This is why it takes longer to work than some other laxatives. Most people have a bowel movement within one to three days of starting it, though it can take up to four days. The standard adult dose is one capful (17 grams) stirred into 8 ounces of water, juice, coffee, or tea, taken once daily.
It Is Not the Same as Ethylene Glycol
A common concern is that polyethylene glycol sounds like it’s related to antifreeze, and there’s a reason for the confusion. Ethylene glycol, the toxic compound in antifreeze, is the single-unit building block that gets chained together to form polyethylene glycol. But the two substances behave very differently in your body.
Ethylene glycol is toxic because your liver breaks it down into harmful byproducts, including glycolic acid and oxalate, which can cause severe kidney damage and metabolic problems. Polyethylene glycol 3350, by contrast, is a long, stable chain of those units bonded together. Your body doesn’t break it apart the same way. PEG 3350 has low toxicity and is widely used not just in laxatives but in pharmaceuticals, food products, and cosmetics. The “3350” refers to its molecular weight, meaning the chain is long enough that it passes through your digestive system largely unchanged.
Use in Children
MiraLAX is labeled for adults 17 and older, but pediatric gastroenterologists frequently recommend polyethylene glycol 3350 for children with constipation. Joint guidelines from the North American and European societies for pediatric gastroenterology list PEG 3350 as the first-line treatment for both clearing impacted stool and ongoing maintenance in children. The typical maintenance dose for kids is weight-based, ranging from 0.2 to 0.8 grams per kilogram of body weight per day, adjusted based on how the child responds.
For children with fecal impaction, the recommended dose is higher, at 1 to 1.5 grams per kilogram per day for three to six days. Studies have found PEG 3350 more effective than lactulose, milk of magnesia, and mineral oil for pediatric constipation. Guidelines recommend continuing maintenance treatment for at least two months and tapering gradually rather than stopping abruptly.
Side Effects and Who Should Be Cautious
At standard laxative doses, PEG 3350 causes minimal shifts in electrolytes like sodium and potassium. During drug development, researchers found that any electrolytes added to laxative-sized doses of PEG 3350 were completely absorbed by the gut, which is why MiraLAX contains no added electrolytes. (By contrast, the much larger PEG-based bowel prep solutions used before colonoscopies do include electrolytes, because the volume involved is dramatically higher.)
That said, certain conditions call for caution. People with kidney disease, inflammatory bowel disease, severe ulcerative colitis, or a history of seizures should talk to a doctor before using it. The same applies if you have heart failure, dehydration, or known electrolyte imbalances like low potassium, sodium, calcium, or magnesium. Anyone with a bowel obstruction, perforation, or severely swollen intestine should not use PEG 3350 at all. Older adults are more likely to have reduced kidney function, which may increase the risk of side effects.
Generic vs. Brand: What You’re Paying For
Because the FDA considers all polyethylene glycol 3350 laxative products identical in active ingredient, strength, and indication, choosing between MiraLAX and a generic comes down to price and preference. The generic versions sitting next to MiraLAX on the pharmacy shelf contain the same 17-gram dose of the same compound. The only differences are packaging, branding, and cost. If your store brand says “polyethylene glycol 3350” and lists 17 grams per dose, you’re getting the same drug.