Polyethylene glycol, commonly called PEG, is a synthetic compound made from ethylene oxide and water. It’s one of the most widely used chemicals in modern life, showing up in laxatives, skin creams, injectable medications, and food packaging. The number after “PEG” on a label (like PEG 400 or PEG 3350) refers to its average molecular weight, which determines whether it’s a liquid or a solid and what it’s used for.
How PEG Works as a Chemical
PEG is a polymer, meaning it’s a long chain of repeating molecular units. It dissolves completely in water, and this water-attracting property is central to nearly everything it does. Lower molecular weight versions (below 600) are liquids at room temperature and work well as solvents in creams and lotions. Higher molecular weight versions (above 600) are waxy solids, making them useful as ointment bases and tablet coatings.
The numbering system is straightforward. PEG 400 has an average molecular weight of 400, PEG 3350 averages 3,350, and so on. As the number climbs, the compound becomes thicker, more solid, and behaves differently in formulations. This flexibility is why PEG appears across so many industries.
The Laxative You’ve Seen at the Pharmacy
PEG 3350 is the active ingredient in popular over-the-counter laxatives like MiraLAX. It works through osmosis: when you dissolve the powder in water and drink it, PEG molecules travel to your colon largely unabsorbed. Once there, they bind and hold onto water molecules, preventing the colon from reabsorbing that fluid as it normally would. Research shows PEG actually sequesters more water than its molecular weight alone would predict, likely because of the way it interacts with water molecules at a chemical level.
The result is softer, more hydrated stool that’s easier to pass. The standard adult dose is 17 grams dissolved in a beverage, taken once daily. Doctors also prescribe higher-volume PEG solutions for bowel preparation before colonoscopies, where the goal is to completely flush the digestive tract.
Common side effects of oral PEG include bloating, gas, nausea, stomach cramps, and diarrhea. These are generally mild and related to the water-drawing mechanism doing its job. In children, the FDA has received reports of neurological and behavioral side effects with prolonged use, including headaches, tremors, mood swings, anxiety, and aggression, though these cases are uncommon.
PEG in Medications and Drug Delivery
Beyond laxatives, PEG plays a critical behind-the-scenes role in how medications work inside your body. A process called PEGylation involves attaching PEG molecules to drugs, proteins, or nanoparticles. This creates a water-attracting shield around the drug that makes it harder for enzymes to break down and harder for the kidneys to filter out. The practical effect: the medication stays active in your bloodstream much longer, which means it works better and you need fewer doses.
Pharmaceutical companies typically use PEG with a molecular weight above 20,000 for this purpose. The thick hydration layer around the PEG-coated drug reduces recognition by immune cells that would otherwise tag it for removal. One PEGylated enzyme replacement therapy for a rare genetic condition, for example, achieves a half-life of roughly 79 hours, meaning it stays active in the body for days rather than hours. This technology has been applied to cancer treatments, where PEGylated drugs can accumulate more effectively in tumors by slipping through the leaky blood vessels that surround them.
Skin Care and Personal Care Products
If you check the ingredient list on your shampoo, lotion, or moisturizer, you’ll likely find some form of PEG. In cosmetics and personal care, PEG serves several roles depending on its molecular weight and what it’s combined with. It acts as an emulsifier (helping oil and water mix), a surfactant (helping products spread and clean), a humectant (drawing moisture to the skin), and a conditioning agent.
PEG derivatives are tailored for specific jobs. PEG castor oils like PEG-40 hydrogenated castor oil show up in lotions, creams, and hair care products as solubilizers and conditioners. PEG fatty acids serve as emulsifiers and dispersing agents in bath oils. PEG amine ethers function as foam boosters and mild detergents in shampoos and hair-dye creams, while also reducing irritation.
Food Packaging and Industrial Uses
PEG doesn’t typically show up as an ingredient in food itself, but the FDA permits its use in materials that come into contact with food, like packaging adhesives and coatings. Under federal regulations, PEG with a mean molecular weight between 200 and 9,500 can be used in food-contact articles, provided it meets purity standards. Specifically, it must contain no more than 0.2 percent ethylene and diethylene glycols (toxic compounds that can form as byproducts) for PEG with a molecular weight of 350 or higher, and no more than 0.5 percent for lower molecular weight versions.
Allergic Reactions to PEG
PEG allergy is rare, but it exists, and it’s been recognized with increasing frequency over the past two decades. This matters because PEG is so ubiquitous. A person allergic to PEG can react to laxatives, injectable medications, skin creams, and even cosmetics, often without realizing the same ingredient is the culprit.
When reactions do occur, they tend to be serious. A review of 37 published cases of immediate PEG reactions found that 76% met the clinical criteria for anaphylaxis, including anaphylactic shock. Common symptoms include itching, flushing, hives, swelling, low blood pressure, and difficulty breathing. In one case series of ten adults diagnosed with PEG allergy, 80% had experienced at least one anaphylactic reaction requiring epinephrine before anyone identified PEG as the trigger. Patients went through a median of three reactions before getting a diagnosis.
The diagnostic difficulty is the real problem. Because PEG appears in so many products under various names (macrogol, polyoxyethylene, cetomacrogol), people who react to it often appear to be allergic to multiple unrelated drugs. One documented patient had anaphylaxis after an injectable contraceptive containing PEG 3350 but tolerated the same hormone in an oral contraceptive pill that didn’t contain PEG. Another reacted to an over-the-counter laxative, a heartburn medication, and specific cosmetics, all of which shared PEG as a common ingredient.
Who Should Avoid PEG
PEG-based laxatives are contraindicated if you have a known or suspected bowel obstruction, appendicitis, inflammatory bowel disease, or a perforated bowel. Anyone with a confirmed hypersensitivity to polyethylene glycol or any component in a PEG-containing formulation should avoid it entirely. Kidney damage has been reported in rare cases involving intravenous drugs that use PEG as a carrier, and in burn patients receiving repeated topical PEG application to damaged skin, so compromised skin barriers and kidney function warrant extra caution.