MiraLAX contains a single active ingredient: polyethylene glycol 3350, a synthetic compound made of repeating units of ethylene glycol linked together into a large molecule. That’s it. The product has no inactive ingredients, no dyes, no flavoring, no preservatives, and no gluten. It’s essentially pure powder in every dose.
What Polyethylene Glycol 3350 Actually Is
Polyethylene glycol 3350 (often shortened to PEG 3350) is a water-soluble polymer, meaning it’s a long chain of identical small chemical units bonded together. The “3350” refers to its average molecular weight, which determines how the compound behaves in your body. At this size, the molecule is too large to be absorbed through the intestinal wall, so it passes through your digestive tract without entering your bloodstream. This is a key reason it’s considered a gentle option for constipation relief.
The powder itself is white, odorless, and tasteless once dissolved. It mixes into liquid without changing the flavor noticeably, which is why the label says you can stir it into any beverage, whether cold, hot, or room temperature.
How It Works in Your Body
PEG 3350 is an osmotic laxative. When you drink it dissolved in liquid, it travels to your colon and draws water into the intestine through osmosis. This extra water softens the stool and increases its bulk, which stimulates your bowel to move things along naturally. Unlike stimulant laxatives that force the muscles of your intestine to contract, PEG 3350 simply changes the water balance in your colon. Your body does the rest.
This mechanism is why MiraLAX doesn’t cause the cramping or urgency that stronger laxatives often do. It’s also why it takes longer to work. Most people have a bowel movement within one to three days after taking it, though some sources suggest it can take up to four days.
What’s in a Standard Dose
Each dose contains exactly 17 grams of PEG 3350 powder. If you’re using the bottle, the cap doubles as a measuring tool: you fill it to the line marked on the white section. Single-use packets also contain 17 grams. You dissolve the powder in 4 to 8 ounces of any beverage and drink it, typically once a day.
Because the product contains no other ingredients, it dissolves clearly and doesn’t change the color or texture of your drink. This makes it one of the more straightforward over-the-counter laxatives to take, especially for people sensitive to artificial flavors or sweeteners found in competing products.
How MiraLAX Became Over-the-Counter
PEG 3350 was first approved by the FDA as a prescription medication in February 1999 under the MiraLAX brand name. The original approval was based on clinical studies showing it was safe and effective for up to two weeks of use. In 2006, the FDA approved the switch from prescription to over-the-counter status, making it available without a doctor’s visit. The formulation didn’t change during this transition. The same single-ingredient powder that required a prescription is what you now pick up off the shelf.
Duration and Safety Limits
The standard guidance is to use MiraLAX for no more than seven consecutive days without checking with a healthcare provider. This isn’t because the ingredient becomes dangerous after a week. It’s because constipation lasting longer than that may signal an underlying issue worth investigating. Some people do use PEG 3350 for longer periods under medical supervision, particularly for chronic constipation, but the over-the-counter label is intentionally conservative.
Because PEG 3350 isn’t absorbed into the bloodstream, it doesn’t interact with most medications the way other laxatives can. It also doesn’t cause electrolyte imbalances at standard doses, which is a concern with some other osmotic laxatives that pull minerals along with water.
Why People Ask About the Ingredients
Part of the reason this question comes up so often is that “polyethylene glycol” sounds industrial. And it is used in industrial applications at different molecular weights. PEG compounds show up in everything from skin creams to antifreeze, depending on the size of the molecule. But PEG 3350 specifically is a large, inert molecule that the body doesn’t break down or absorb. The version used in MiraLAX meets pharmaceutical-grade purity standards (designated “NF,” meaning it conforms to the National Formulary). It’s not the same product used in other applications, even though the base name is similar.
If you have concerns about sensitivities, the absence of inactive ingredients is worth noting. Many competing laxative powders contain sweeteners, flavoring agents, or coloring. MiraLAX contains none of these, making it one of the cleanest formulations available in the laxative aisle.