Generic Miralax works just as well as brand-name Miralax. Both contain the exact same active ingredient, polyethylene glycol 3350, at the same 17-gram dose, and neither product contains any inactive ingredients or fillers. This makes Miralax one of the simplest cases in the generic-vs-brand debate: there is literally nothing different inside the bottle.
Why They’re Identical
Miralax is a single-ingredient product. The active ingredient is polyethylene glycol 3350 (often abbreviated PEG 3350), a powder you dissolve in liquid. The brand-name version lists no inactive ingredients at all, and neither do the generic versions. That means whether you pick up Miralax, ClearLax, GaviLAX, or a store-brand equivalent, you’re getting the same white powder with nothing else added.
Many generic drugs differ from their brand-name counterparts in fillers, coatings, or dyes. Those differences are cosmetic and don’t affect how the drug works, but they occasionally matter for people with specific allergies or sensitivities. With PEG 3350 products, even that small variable is off the table.
What the FDA Requires
Before any generic reaches pharmacy shelves, the FDA requires it to match the brand-name drug in active ingredient, strength, dosage form, route of administration, and bioequivalence. Bioequivalence means the generic delivers the same amount of the active ingredient into your body at the same rate. Generic manufacturers must also meet the same current Good Manufacturing Practice (cGMP) standards that brand-name facilities follow, and the FDA inspects both types of facilities on an ongoing basis.
For a product like PEG 3350, which is a simple powder dissolved in water, bioequivalence is straightforward. There’s no special coating to engineer, no timed-release mechanism to replicate, and no complex formulation to reverse-engineer. The generic is, for all practical purposes, the same product in a different container.
Clinical Evidence
Head-to-head trials comparing different PEG formulations in adults with constipation have consistently shown comparable efficacy and safety. Studies in children have reached similar conclusions: different PEG products perform equally well, with the main differences showing up only when electrolytes are added to the formula (which is a separate product used for bowel preparation, not the over-the-counter laxative you’d buy for everyday constipation).
The side effect profile is the same regardless of which version you take. The most common effects are nausea, bloating, cramping, and gas. Less commonly, some people experience diarrhea, which usually signals you should reduce the dose or stop use temporarily.
How PEG 3350 Works
PEG 3350 is an osmotic laxative, meaning it pulls water into your colon to soften stool and stimulate a bowel movement. It isn’t absorbed into the bloodstream in any meaningful amount, which is one reason the side effect profile is mild and the generic-vs-brand question is so straightforward. The powder doesn’t need to be metabolized by your liver or reach a specific concentration in your blood. It just needs to reach your intestines and attract water, which any source of PEG 3350 will do identically.
The standard adult dose is one capful (17 grams) dissolved in 4 to 8 ounces of any beverage, once daily. Most people see results within one to three days. The product has no taste or odor when dissolved, regardless of brand.
The Only Real Difference: Price
Brand-name Miralax typically costs two to three times more than store-brand generics for the same number of doses. Since the powder inside is chemically identical and contains no additional ingredients in either version, the price gap reflects brand recognition and marketing rather than any difference in quality or performance. Warehouse clubs and pharmacy chains frequently sell large containers of generic PEG 3350 at a significant discount per dose compared to the Miralax label.
If you’ve been using brand-name Miralax and are considering switching, there’s no medical reason to hesitate. You can switch back and forth between any PEG 3350 product without affecting how well it works or how your body responds to it.