Polyethylene glycol (PEG) is a water-soluble compound used across medicine, skincare, food production, and pharmaceuticals. Its most familiar use is as an over-the-counter laxative for constipation, but the same basic molecule shows up in everything from colonoscopy prep drinks to moisturizers to drug capsules. The specific application depends on the molecular weight of the PEG, which determines whether it’s a liquid, a waxy solid, or something in between.
PEG as a Laxative for Constipation
The most common medical use of polyethylene glycol 3350 is treating occasional constipation in adults and older teens. It’s classified as an osmotic laxative, meaning it works by pulling water into the colon and keeping it there. That extra water softens stool and increases the frequency of bowel movements, making them easier to pass. The standard adult dose is 17 grams once daily, typically dissolved in a glass of water, juice, or another beverage.
What makes PEG particularly effective is that it binds water molecules more strongly than its molecular size would suggest. Lab measurements show that PEG essentially sequesters water from the surrounding solution, creating a stronger osmotic pull than scientists originally predicted. Your body doesn’t absorb higher-molecular-weight PEG in any meaningful amount. It passes through the digestive tract doing its job and then leaves.
PEG 3350 is also used in children with chronic constipation, including those younger than two years old. A systematic review published in the Journal of Pediatric Gastroenterology and Nutrition found no reported side effects in children under 24 months, though established dosing guidelines for that age group don’t yet exist. Pediatric dosing is typically determined by a child’s weight and managed by their doctor.
Bowel Preparation Before Colonoscopy
If you’ve ever had a colonoscopy, there’s a good chance your prep solution contained PEG. Traditional bowel prep formulas combine PEG 3350 with electrolytes (sodium, potassium, and bicarbonate) to flush the colon clean before the procedure. These solutions have a reputation for being unpleasant, partly because older formulations required drinking about 4 liters of liquid.
Newer approaches have reduced that volume significantly. Some protocols use about 204 grams of PEG powder dissolved in roughly one liter of water, taken in three divided doses an hour apart with additional water between each dose. That’s about a quarter of the liquid volume compared to traditional large-volume preparations, which makes the experience considerably more tolerable. Patients typically finish the prep the evening before their colonoscopy, regardless of whether the procedure is scheduled for the morning or afternoon.
Common Side Effects
PEG-based laxatives and prep solutions can cause nausea, bloating, stomach cramps, and a feeling of fullness. Some people experience heartburn, weakness, or chills. These effects are generally mild and temporary. People with intestinal blockages, perforations in the stomach or intestinal lining, or conditions that impair stomach emptying should not take PEG products.
How Molecular Weight Changes the Product
PEG isn’t a single substance. It’s a family of compounds available in molecular weights ranging from 200 to 10,000,000 grams per mole. That number after “PEG” (200, 400, 3350, 6000) refers to the average molecular weight, and it completely changes the physical properties and uses of the compound.
Lower-molecular-weight PEGs (200 to 600) are clear liquids at room temperature. PEG 300 and PEG 400 are widely used as cosolvents in injectable and oral drug formulations, helping dissolve medications that don’t mix well with water on their own. These smaller PEGs are well absorbed by the digestive tract, with more than 90% excreted unchanged in urine and feces. As molecular weight increases, absorption drops. PEG 1000 and above are semi-crystalline solids, and PEG 3350, the laxative version, passes through the gut largely unabsorbed, which is exactly why it works as an osmotic agent. Grades above 20,000 are often called polyethylene oxides and are used in controlled-release drug formulations.
Uses in Skincare and Cosmetics
PEG and its derivatives are staple ingredients in personal care products. Because many forms of PEG attract and hold water, they function as humectants (ingredients that keep skin moisturized) and skin conditioners. Different PEG compounds serve different roles in a single product: surfactants that help cleansers foam, emulsifiers that keep oil and water from separating in lotions, and solubilizing agents that dissolve fragrances, vitamins, and other active ingredients into water-based formulas.
PEG-40 hydrogenated castor oil, for example, is used to dissolve essential oils and perfume compositions into aqueous solutions. PEG-20 glyceryl triisostearate works as an emulsifier and dispersing agent. These aren’t exotic exceptions. PEG-based ingredients appear in a wide range of everyday products, from face washes to shampoos to sunscreens. Their water-attracting properties also make them effective penetration enhancers in topical skin treatments, helping active ingredients absorb more efficiently.
Food and Pharmaceutical Applications
Certain PEG forms are permitted as food additives. Polysorbates (which are polyoxyethylene sorbitan esters) and PEG with an average molecular weight of 6,000 appear in various processed foods, typically as emulsifiers or stabilizers that maintain texture and consistency.
In pharmaceutical manufacturing, PEG plays a behind-the-scenes role in how medications are built. It serves as a base for ointments, a vehicle for drugs inside capsules, a binding agent in tablets and pills, and a component of suppositories and liquid prescriptions. It’s also used in veterinary medicine across injectable, topical, eye, oral, and rectal preparations. PEG’s versatility comes from its adjustable physical properties, its compatibility with biological tissue, and the fact that the body handles it without significant toxicity at the molecular weights used in these products.