Poly glycol most commonly refers to polyethylene glycol (PEG), a synthetic, water-soluble compound made from repeating units of ethylene oxide. It shows up in an enormous range of products, from over-the-counter laxatives to skin creams to industrial lubricants. The term can also refer to propylene glycol, a related but chemically distinct substance. Here’s what each one is, how they’re used, and what you should know about safety.
Polyethylene Glycol: The Basics
Polyethylene glycol is a chain-like molecule built from repeating segments of ethylene oxide bonded together. Its chemical formula is H(OCH₂CH₂)ₙOH, where “n” represents the number of repeating units. That number can vary widely, and the result is a whole family of PEG compounds ranging from thin liquids to waxy solids.
Low molecular weight PEGs (under about 600) are clear, viscous liquids at room temperature. PEG 300, for example, has a melting point of roughly negative 11°C and mixes easily with water. Higher molecular weight versions (above 600) become increasingly solid and wax-like. This flexibility is a big part of why PEG appears in so many different products: manufacturers can choose the exact version that suits their needs.
PEG’s most important chemical trait is its ability to form hydrogen bonds with water molecules. This lets it attract and hold onto water, which explains its usefulness as both a moisture-retaining ingredient in skin products and as the active mechanism in laxatives.
How PEG Is Used in Medicine
The most familiar medical use of PEG is as an over-the-counter laxative, sold under brand names like MiraLAX. The specific version used is PEG 3350, a powder you dissolve in liquid. It works as an osmotic laxative, meaning it holds water in the stool rather than letting the intestines reabsorb it. This softens the stool and increases the frequency of bowel movements. A typical dose is about one heaping tablespoon (17 grams) of powder mixed into a beverage, taken once daily for up to two weeks.
PEG also plays a behind-the-scenes role in many medications. It helps dissolve active ingredients that wouldn’t otherwise mix well with water, and it’s used as a coating or filler in tablets and capsules. In more specialized medical settings, higher-concentration PEG solutions are used for bowel preparation before colonoscopies.
PEG in Skin Care and Cosmetics
PEG is a common ingredient in creams, lotions, shampoos, and other personal care products, where it serves several different functions depending on the formulation. As an emollient, it creates a barrier on the skin that prevents moisture loss. As a humectant, it actively pulls water toward the skin and releases it gradually, providing longer-lasting hydration. It also works as an emulsifier, helping oil-based and water-based ingredients blend smoothly together.
Different molecular weights serve different cosmetic purposes. Lighter PEGs work as solvents in liquid formulations like creams and lotions, while heavier, solid PEGs are better suited as ointment bases. You’ll often see PEG listed on ingredient labels with a number after it (PEG-40, PEG-60, etc.), which indicates the molecular weight and gives formulators precise control over texture and function. Modified versions like PEG castor oils appear in hair care products as conditioners and solubilizers.
Industrial and Food Applications
Beyond medicine and cosmetics, PEG serves as a plasticizer, lubricant, and surfactant in a range of industrial processes. It’s used in wood preservation, where its water-attracting properties help stabilize waterlogged archaeological wood. In manufacturing, its low toxicity and chemical stability make it useful as a base for products where safety is a concern.
PEG is also an FDA-approved food additive. It appears in food-related applications as an anti-foaming agent, a coating material, and a processing aid. The FDA lists PEG across multiple food additive regulations covering molecular weights from 200 to 9,500.
How PEG Differs From Propylene Glycol
People sometimes confuse polyethylene glycol with propylene glycol, but they’re different compounds. Propylene glycol has a much simpler structure (C₃H₈O₂) and a lower boiling point of about 188°C, compared to PEG, which decomposes above 200°C. Propylene glycol also has a lower flash point, meaning it catches fire more easily and requires more careful handling and storage.
Their applications overlap in some areas but diverge in others. Propylene glycol is the better choice for industrial solvents and antifreeze because it stays liquid over a wide temperature range and evaporates slowly. PEG is generally preferred for pharmaceutical and cosmetic formulations. If you see “propylene glycol” on a food label, it’s being used as a humectant or solvent. If you see “PEG” followed by a number, that’s polyethylene glycol.
Safety Profile
PEG is recognized by the FDA as safe for use in food, drugs, and cosmetics. It has a long track record, and the body does not significantly absorb or metabolize it when taken orally at standard doses. When used as a laxative, it passes through the digestive tract largely unchanged.
Allergic reactions to PEG are rare but can be severe when they do occur. A review of 37 reported cases of immediate allergic reactions to PEG, published between 1977 and 2016, found that 76% met the clinical criteria for anaphylaxis. Symptoms included itching, hives, swelling, low blood pressure, and difficulty breathing. In a separate case series, four out of five confirmed PEG-allergic patients experienced anaphylaxis to medications containing PEG, including one near-fatal episode involving cardiac arrest. These cases are uncommon enough that most people will never experience a reaction, but the severity of documented reactions means that anyone who has had an unexplained allergic reaction to a medication or personal care product should consider PEG as a possible trigger.
PEG allergy has drawn more attention in recent years partly because PEG is used as a stabilizer in some injectable medications, including certain vaccines. For people with a confirmed PEG allergy, this is relevant information to share with a healthcare provider before receiving any injection.