Teeth whitening strips contain a peroxide-based bleaching gel pressed onto a thin, flexible plastic backing. The active ingredient is either hydrogen peroxide or carbamide peroxide, typically at lower concentrations than what a dentist would use in-office. The rest of the strip is a mix of adhesive polymers, moisturizers, and flavoring agents designed to keep the gel in contact with your teeth long enough for the peroxide to work.
The Active Ingredient: Peroxide
Every whitening strip that actually changes tooth color relies on some form of peroxide. Most over-the-counter strips use hydrogen peroxide, usually in concentrations between 6% and 14%. Some use carbamide peroxide instead, which breaks down into hydrogen peroxide and urea once it contacts moisture in your mouth. Carbamide peroxide is roughly one-third as potent, so a 10% carbamide peroxide gel delivers the equivalent bleaching power of about 3.5% hydrogen peroxide.
The concentration matters because it determines both how quickly your teeth whiten and how likely you are to experience sensitivity. Professional treatments can use hydrogen peroxide at 25% to 40%, which is why they produce faster results. Over-the-counter strips use lower concentrations and compensate by requiring repeated daily applications over one to two weeks.
How Peroxide Actually Whitens Teeth
Peroxide whitens teeth by oxidizing the organic material inside enamel, not by stripping or etching the tooth surface. Under the right conditions, hydrogen peroxide generates free radicals and reactive oxygen molecules. These reactive molecules target chromophores, the pigmented compounds responsible for staining. They break apart the chemical bonds (particularly double bonds) in those molecules, converting them into smaller, less pigmented fragments that reflect less light.
The result is a tooth that appears brighter, even though the mineral structure of the enamel itself stays largely unchanged. Research published in the Journal of Dentistry confirmed that hydrogen peroxide does not significantly alter the ratio of organic to inorganic content in enamel. It whitens by chemically reshaping the organic matrix rather than dissolving it.
The Plastic Strip Itself
The strip is a thin piece of polyethylene, the same type of flexible plastic used in food packaging. It serves as a carrier for the gel and presses it against your teeth. Most strips are shaped to cover the front six to eight teeth on either the upper or lower arch. The plastic is porous enough to let oxygen escape as the peroxide breaks down, which prevents the strip from bubbling off your teeth during use.
Adhesive and Thickening Agents
The gel needs to stay put on a wet, curved tooth surface for 30 to 60 minutes. Several inactive ingredients make that possible:
- PVP (polyvinylpyrrolidone): A water-soluble polymer that helps the gel film adhere to enamel. It’s widely used in adhesives, contact lens solutions, and pharmaceutical coatings.
- Carbomer or sodium polyacrylate: Thickening agents that give the gel its viscosity so it doesn’t slide off your teeth or get immediately diluted by saliva.
- PVM/MA copolymer: Another adhesive polymer that improves grip on the tooth surface, commonly found in denture adhesives and hair products.
- Ethylcellulose: A film-forming agent that helps create a stable, even layer of gel across the strip.
These polymers are all generally recognized as safe for oral use and wash away with saliva and rinsing after you remove the strip.
Moisturizers and Solvents
Glycerin is one of the most common inactive ingredients in whitening strips. It serves as a humectant, keeping the gel moist during storage and application so the peroxide doesn’t degrade before use. Water is the other primary solvent, and a small amount of alcohol sometimes appears in the formula to help dissolve other ingredients and improve the gel’s texture.
Flavoring and Comfort Ingredients
Whitening strips often include menthol for a mild minty flavor, making the experience more tolerable during the 30 to 60 minutes the strips sit on your teeth. Some formulations add xylitol, a sugar alcohol that provides slight sweetness without promoting tooth decay. Xylitol also has a mild cooling effect that complements the menthol.
A few strips include sodium citrate or potassium citrate as buffering agents. These help control the pH of the gel, keeping it within a range that balances bleaching effectiveness with comfort. If the gel is too acidic, it can irritate gum tissue. Too alkaline, and the peroxide breaks down too quickly to be useful.
Desensitizing Ingredients
Tooth sensitivity is the most common side effect of peroxide bleaching, so some strips add ingredients meant to reduce it. Potassium nitrate and sodium fluoride are the two most frequently used desensitizers. Potassium nitrate works by calming the nerve inside the tooth, reducing its ability to fire pain signals. Sodium fluoride helps remineralize enamel and block the tiny tubules in dentin that transmit sensation.
Some strips also include hydroxyapatite, a synthetic form of the mineral that makes up most of your tooth enamel. The idea is that hydroxyapatite particles fill microscopic gaps in the enamel surface, reducing sensitivity and protecting the tooth during bleaching. Whether these desensitizing additives make a meaningful clinical difference is debatable. A randomized triple-blind clinical trial found that adding 3% potassium nitrate and 0.2% sodium fluoride to a carbamide peroxide bleaching gel did not actually reduce tooth sensitivity compared to the same gel without them.
What’s Not in the Strips
Whitening strips do not contain abrasives. Unlike whitening toothpastes, which often rely on silica or baking soda to physically scrub surface stains, strips work entirely through chemical oxidation. They also don’t contain chlorine dioxide, a bleaching agent found in some questionable “whitening” products sold online that can erode enamel. Legitimate strips from established brands stick to peroxide as the sole bleaching agent, which is the only compound with strong evidence for changing the intrinsic color of teeth rather than just removing surface buildup.