How peroxide changes a tooth’s color
Many tooth stains come from colored molecules lodged in or beneath the enamel, the hard outer covering of a tooth. Hydrogen peroxide can break some of these molecules into smaller, less visibly colored compounds through oxidation—a chemical reaction that alters their structure. Whitening strips hold peroxide against the teeth long enough for some of it to pass through enamel and act on stains below the surface.
Why whitening can make teeth feel sensitive
Enamel is not a sealed shell. Beneath it lies dentin, a layer threaded with microscopic channels that lead toward the tooth’s pulp, where nerves and blood vessels sit. Peroxide can move through enamel and dentin; during whitening, that exposure can temporarily make the nerve more reactive to cold, air, or touch. The sting is a response to irritation, not a sign that the stain is being pulled out of the tooth.
Sensitivity varies. Teeth with existing wear, exposed roots, cracks, or other irritation may be more reactive, and using a product more often or longer than its directions call for can increase exposure. Whitening does not affect everyone the same way, so a strip that feels comfortable for one person may bother another.
What the discomfort can—and cannot—tell you
Brief sensitivity often eases after whitening stops, but sharp, worsening, or lingering pain is not something to push through. It can have causes unrelated to whitening, such as a cavity or a damaged tooth, and merits dental advice. The practical tradeoff is that peroxide needs contact with teeth to change stain molecules, while the same ability to move through tooth layers can temporarily make some teeth more responsive to everyday cold.
