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· Treatments, Explained

How Tooth Bleaching Works

By Dr. Jill Kollar, Precision Dental of Windsor

Title graphic reading How Tooth Bleaching Works, beside a clear whitening tray, a gel syringe, and a tooth model on a counter in a dental office

“Whitening” is used for two different jobs, and only one of them changes the color of a tooth.

The first job is removing stain from the surface. Whitening toothpastes and the polishing at a cleaning work this way. They take off the film and surface discoloration that build up from coffee, tea, wine, and tobacco. The tooth underneath is the same color it was before. Some products on a drugstore shelf go further: whitening strips and some gels contain peroxide, and those do bleach.

The second job is bleaching. Bleaching changes the color of the tooth itself, from the inside. The whitening systems we use rely on peroxide to do it. Everything in this post is about how that works.

The short answer. Bleaching changes the color of the tooth itself. Peroxide passes through the enamel and alters the colored compounds in the dentin, so the tooth reflects more light. The change builds over repeated sessions, fillings and crowns do not lighten along with the tooth, and sensitivity during treatment is common.

Where a tooth’s color comes from

A tooth has two layers that matter here. Enamel, the outer layer, is hard and mostly translucent, like frosted glass. Under it sits dentin, which is yellowish and is where most of a tooth’s visible color comes from. You see dentin color through the enamel.

Teeth look darker over time for two reasons. Enamel thins slowly with years of use, so more of the dentin shows through. And colored compounds from food, drink, and tobacco work their way into the tooth and collect in the dentin. Those collected compounds are called chromogens: large molecules that absorb light, which is why the tooth looks darker instead of reflecting light back.

The chemistry, step by step

Bleaching gel is placed against the teeth in a tray. Then:

  1. The peroxide in the gel passes through the enamel. Enamel is not solid in the way it looks; peroxide molecules are small enough to travel through it and reach the dentin, and peroxide can pass through the tooth within minutes. One session starts the process. The color change builds over repeated sessions, which is why home bleaching runs over days or weeks.
  2. Inside the tooth, the peroxide breaks down and produces reactive substances.
  3. Those substances change the parts of the chromogens that absorb light, so the compounds absorb less light and look pale. Researchers still debate the exact steps. The result is not debated: the compounds absorb less light.
  4. With the colored compounds altered, the dentin reflects more light, and the tooth looks whiter. Small changes at the enamel surface, which change how it reflects light, can contribute as well.

Nothing is coated, painted, or sanded. Bleaching does not drill away or cover healthy tooth structure. Its main work is chemical, on the colored compounds inside the tooth.

Two peroxides, and why the percentages look different

Bleaching gels use one of two forms of peroxide.

Hydrogen peroxide is the active ingredient itself. It works quickly and is used up quickly, so it suits shorter sessions.

Carbamide peroxide is hydrogen peroxide combined with urea. On the tooth it breaks down into hydrogen peroxide and urea, and it releases its peroxide slowly over several hours. That slow release is why carbamide peroxide is used for overnight wear.

The percentages are not directly comparable. Carbamide peroxide yields about one third of its weight as hydrogen peroxide, so a gel labeled 10% carbamide peroxide delivers about 3.5% hydrogen peroxide. A 16% carbamide peroxide gel delivers about 5.5%. When you compare products, compare what they deliver, not the number printed largest on the box.

This is also why wear times differ by product and strength. Our aftercare page lists the schedules for the systems we use: KöR Whitening and Opalescence, including the KöR-Night schedule of a minimum of 14 consecutive nights. That schedule comes from the KöR protocol for its system. The general pattern is the same across systems: each session works on more of the pigment, and the sessions add up.

Why some teeth respond slowly

Most teeth lighten steadily. Some do not, and the reason is usually where the color sits and what caused it.

Gray discoloration, including the banded staining from tetracycline antibiotics taken while the teeth were forming, is less predictable. It often improves, but it can take a longer supervised course. In one clinical trial, tetracycline-stained teeth were bleached nightly for up to six months, and most reached a result the evaluators rated excellent or satisfactory. Yellow-brown discoloration usually responds faster than gray.

A single dark tooth is a different problem. A tooth that darkens after an injury or after the nerve inside it dies is discolored from the inside. That tooth needs to be examined first. Bleaching gel also reaches into the tooth, but whether it helps depends on the cause and on the condition of the pulp, and treatment may involve a specialist.

What bleaching cannot change

Restorative materials do not whiten. Fillings, crowns, and veneers keep the shade they were made at:

  • Crowns and veneers
  • Tooth-colored fillings and bonding
  • Denture teeth

Those materials were made to match your teeth at the shade they were when the work was placed. If the natural teeth around them lighten, the dental work keeps its original shade and can start to stand out.

This affects the order of work when the treatment is planned and elective. If you are planning whitening and also a crown or a visible filling that will be matched to your tooth color, whitening first can help with shade selection. Urgent problems are different: decay, a leaking filling, or other treatment that should not wait should not be postponed for whitening.

After bleaching, two different waits apply. Adhesive work, such as bonding and tooth-colored fillings, waits at least 7 to 10 days, because fresh peroxide in the tooth interferes with how bonding materials stick. The final shade is checked again at about two weeks, when the color has stabilized, so new work is matched to the stabilized shade.

Why teeth get sensitive during bleaching

The pathway that lets peroxide reach the dentin also lets some of it reach the pulp, the tissue with the nerve at the center of the tooth. That irritation is the sensitivity people feel, usually as a short zing with cold air or cold drinks.

Sensitivity is usually temporary. Pain that is severe, or that does not settle, should be checked. One caution from the research: in small studies, concentrated peroxide used in the office injured the pulp of some teeth. Home gels are much weaker, and this practice does not offer in-office whitening. That research is why sensitivity should not just be pushed through. The American Dental Association lists temporary tooth sensitivity and gum irritation as the common side effects of whitening. A 2025 review of home whitening studies found that the highest concentrations brought more sensitivity, and that sensitivity was mild on average at every concentration.

Some gels include ingredients meant to help. The Opalescence PF gels we use contain potassium nitrate and fluoride: potassium nitrate calms the nerve response, and fluoride supports the enamel surface. They help many patients. They do not help everyone. Follow your system’s instructions, and if discomfort is more than mild, keeps returning, or gets worse, stop and call us rather than pushing through. Our aftercare instructions cover the details.

Does bleaching damage enamel?

Bleaching does not drill away or cover healthy tooth structure, and it does not remove enamel. Laboratory studies do find small changes at the enamel surface with some gels, such as slight mineral loss. The findings depend on the product and the conditions used. A change a laboratory can measure is not the same as damage that matters in a mouth. A 2026 review of those laboratory studies found a small average drop in enamel hardness, and its authors judge real harm unlikely when gels are used as directed. Product, concentration, and exposure time matter, which is why we assess your teeth first and give you specific instructions.

Gel acidity is part of this. The gels we use are made to sit near neutral pH. Opalescence PF measures about 6.5, and KöR gels are formulated at a neutral pH and kept refrigerated so they stay there. Products sold over the counter vary much more. In a study that measured 26 whitening products, the over-the-counter products ran from pH 5.09 to 11.13, and whitening toothpastes went as low as 4.22. The acidic end is the concern in the laboratory research, where acidic gels produced more enamel surface change and let more peroxide reach the pulp. That is the difference between bleaching and the abrasive approach: a highly abrasive toothpaste removes stain by wearing at the surface, which is real wear, and it is why we compare toothpaste abrasiveness in our post on toothpaste, mouth rinse, and floss.

What bleaching does require is that the gel stays on the teeth and off the gums as much as possible, because peroxide irritates soft tissue. A tray made from a scan of your teeth fits closely, which keeps the gel where it belongs. That fit is most of what a professionally made tray is for.

Why the result fades

Bleaching breaks down the pigment that is in the tooth today. It does not stop new pigment from arriving. Coffee, tea, red wine, and tobacco keep delivering chromogens, and the tooth slowly takes them up again. Most people find the color holds well for months and drifts over years. A short maintenance course with the same trays is usually enough to bring it back, which is why we have patients keep their trays.

My takeaway as a dentist

Bleaching is a conservative cosmetic option. It changes color without removing tooth structure, without covering the tooth, and without committing you to maintaining a restoration for the rest of your life. Its limits are just as clear: it does not lighten dental work to match, it is less predictable on gray stain, and it asks for days of wear rather than one appointment.

If you are thinking about it, the useful first step is an exam, so we can tell you which kind of discoloration you have, what is likely to respond, and whether any planned dental work should be sequenced around it. Our whitening prices page lists the fee for each system and what is included. Let’s talk.

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