Peroxide lightens by oxidizing organic color, not by bleaching the mineral

A stained tooth looks darker because large conjugated organic molecules — chromogens from coffee, tea, wine, tobacco, and food — sit in the porous structure of enamel and dentin. Carbamide or hydrogen peroxide releases reactive oxygen species that oxidize those molecules into smaller, less colored compounds. The mineral crystals of enamel (hydroxyapatite) are not the intended target.

Kwon and Wertz summarized the literature in 2015: "Tooth whitening is a dynamic process that involves diffusion of the whitening material to interact with stain molecules and also involves micromorphologic alterations on the surface and changes within the tooth that affect its optical properties. The interaction seems not to be limited to stain molecules, but rather an affinity-based interaction process that also accompanies effects on sound enamel and dentin structures."

Diffusion comes first. Peroxide moves through enamel pores and the dentinal tubules. Once there, it oxidizes organic chromogens. Surface changes and shifts in optical properties follow. The review noted that the process is not limited to the stain molecules alone.

A 2025 study by Eric and colleagues examined 30 % hydrogen peroxide on extracted human teeth. They reported: "HP treatment lightened tooth shade and significantly reduced enamel microhardness over time. This could be related to changes in surface area and porosity caused by denaturalization of the enamel proteins." Circular dichroism and dynamic light scattering showed shifts in protein conformation and size. BET analysis indicated increased surface area and altered pore volume.

Their clinical significance statement reads: "Our findings showed that HP induced denaturation of enamel proteins, resulting in increased enamel porosity and reduced microhardness. These changes in enamel properties could help explain clinical complications observed with these treatments such as increased sensitivity."

This was an in-vitro study. Fifty extracted adult teeth were incubated with 30 % hydrogen peroxide or with distilled water as a control, then measured with a spectrophotometer and a Vickers hardness tester at several time points. In the short term the peroxide group showed higher lightness, but one week after treatment some of the initial gain in lightness had been partially lost. BET analysis found increased enamel surface area with a reduced average pore size. The study tested one high concentration on extracted teeth, so it cannot tell us how lower-concentration, shorter-contact home use behaves in a mouth with saliva. A laboratory incubation is not a prediction for any particular person.

Joiner reviewed the literature on peroxide and enamel and dentine up to the end of 2006. The majority of the studies reviewed indicated no significant deleterious effects on surface morphology, chemistry or microhardness. The contrasting studies that did show effects generally had limitations: in vitro methods that do not accurately reflect the in vivo situation, or products with a particularly low pH, where acidic erosion was likely to explain the changes. That review predates the 2025 study, and its author was affiliated with a consumer oral-care company, so it is best read alongside newer work rather than instead of it.

The practical reading is straightforward. Peroxide’s job is to oxidize organic color. It can also affect enamel proteins and porosity, especially at higher concentrations and longer exposures. That is one reason short, completed sessions are preferred over abandoned long ones, and why people with a history of sensitivity start with the shortest useful contact.

Restorations do not change shade because they lack the organic chromogen matrix that peroxide targets. Pellicle reforms after a session and can re-bind dietary chromogens; maintenance is mechanical and habitual, not a failure of the oxidation step.

Kwon and Wertz also note that the American Dental Association Council on Scientific Affairs recommends supervision by dental health professionals for whitening. If you have existing restorations, gum concerns, or a history of sensitivity, that is a good reason to ask your dentist first. None of this is dental advice. Decay, active pain, or gum disease means see a dentist before whitening. Shade change varies with starting color and daily habits.

Sources

1. Kwon SR, Wertz PW. Review of the Mechanism of Tooth Whitening. J Esthet Restor Dent. 2015.
2. Eric J et al. The effect of Hydrogen Peroxide treatments on dental enamel porosity and protein structure and its long-term implications on tooth hardness and optical properties. J Dent. 2025.
3. Joiner A. Review of the effects of peroxide on enamel and dentine properties. J Dent. 2007.