Teeth Whitening: What Peroxide Actually Does

Reviewed 31 August 2026. The claims below rest on three named reviews of the whitening literature — a widely cited 2006 literature review of bleaching, a 2015 review of the mechanism, and a 2003 critical review of the biological safety aspects. Written for people comparing whitening options and for clinicians. Not medical advice.

The short answers

Tooth whitening works by chemistry, not by abrasion: peroxide diffuses through enamel and oxidises the coloured molecules, with much of the effect happening in the dentin rather than on the surface (Joiner et al., J Dent 2006-08-01, PMID 16569473) (Kwon et al., J Esthet Restor Dent 2015-09-01, PMID 25969131).
Two variables dominate the result — concentration and time. Higher concentrations work faster; lower concentrations can approach the same effect if worn longer. “Gentle overnight” and “35% in an hour” can converge on a similar shade; the trade is comfort (Joiner et al., J Dent 2006-08-01, PMID 16569473).
Sensitivity is not a rare side effect. In the critical review of the biological literature, tooth sensitivity is described as a common side effect of external bleaching, observed in 15% to 78% of patients, and the same review found that clinical studies on other adverse effects are lacking (Dahl et al., Crit Rev Oral Biol Med 2003-01-01, PMID 12907697).
Light and laser activation: the evidence is thinner than the price suggests. The 2006 review states plainly that data on light-activated systems versus non-light-activated controls in clinical studies is “limited and conflicting” (Joiner et al., J Dent 2006-08-01, PMID 16569473).
Over-the-counter products are not automatically mild, and professional oversight is the recommendation of the mechanism review, which states that supervision by dental professionals “as recommended by the American Dental Association Council on Scientific Affairs is critical to achieving a successful and safe whitening outcome” (Kwon et al., J Esthet Restor Dent 2015-09-01, PMID 25969131).
A concrete, quotable safety line from the 2003 review: the authors recommend avoiding concentrations above 10% carbamide peroxide for external bleaching, because their risk assessment did not find a sufficient safety margin in certain regimens — bleaching a full arch with 35% carbamide peroxide, several daily applications of 22%, or both arches simultaneously with 22% (Dahl et al., Crit Rev Oral Biol Med 2003-01-01, PMID 12907697).

What “whitening” actually does to a tooth

Natural tooth colour is the product of the enamel’s thickness and translucency showing the dentin through it, plus chromophores — coloured molecules — inside both tissues. Peroxide (hydrogen peroxide, or carbamide peroxide that decomposes into it) diffuses into the hard tissues and oxidises those chromophores, converting larger coloured molecules into smaller, less coloured ones. The result is not “a thinner enamel”, but a change in how the tooth absorbs and scatters light; that is why the process is often described as dynamic, involving diffusion, interaction with stain molecules and micromorphological change to the surface and within the tooth, which alters optical properties (Kwon et al., J Esthet Restor Dent 2015-09-01, PMID 25969131).

Two consequences people do not expect. First, the effect is not uniform: the review of mechanisms stresses that the interaction is not limited to stain molecules and that accompanying effects on sound enamel and dentin structures are part of the process — the same reason transient dehydration after an in-office session makes teeth look dramatically whiter on the way out of the chair, then settle back over a day as the tooth rehydrates. Second, the ceiling is biological. Extracted teeth, initial colour and stain type matter, and the older literature is candid that alternative bleach systems to peroxide have received “only minor attention” (Joiner et al., J Dent 2006-08-01, PMID 16569473).

What the reviews say about efficacy

The 2006 review of the literature — still one of the most cited general sources in this area — is the clearest summary of the physics of the offer: the key factors determining whitening efficacy with peroxide products are concentration and time; higher concentrations are faster, while lower concentrations can approach the efficacy of higher ones with extended treatment time. It also names the other modifiers: type of stain, initial tooth colour and the age of the patient (Joiner et al., J Dent 2006-08-01, PMID 16569473).

That has a direct commercial implication. Any product can be made to look equivalent in a comparison if it just wears long enough, and any clinic can sell a faster result with a higher concentration. The meaningful differences between offers are therefore the ones nobody advertises: how much gingival irritation your gums will take at that concentration, how well the tray fits so the gel sits on teeth and not on tissue, and whether the whitening is being done on teeth that have already been examined for the things that make bleaching a bad idea.

On the aesthetic-industry favourite — light activation — the review’s verdict is understated but firm: evidence from clinical studies comparing light-activated systems with non-light-activated controls is “limited and conflicting” (Joiner et al., J Dent 2006-08-01, PMID 16569473). If a clinic prices a lamp into your package, you are being charged for a technology whose comparative advantage in the reviewed literature is not established.

Sensitivity, and the honest safety picture

The most useful safety summary is the 2003 critical review of the biological aspects, because it does not confine itself to marketing outcomes. Its numbers and statements, as published (Dahl et al., Crit Rev Oral Biol Med 2003-01-01, PMID 12907697):

With external bleaching, the first subjective colour change is often visible after 2–4 nights, and more than 90% of cases were reported as satisfactory results.
Tooth sensitivity is a common side effect of external bleaching, observed in 15% to 78% of patients — a range so wide that it tells you the honest answer is “often, and it varies by person and protocol” — and clinical studies addressing the risk of other adverse effects were, at the time, lacking.
For internal bleaching of non-vital teeth, more than 90% immediate success was reported, but over a follow-up of 1 to 8 years, 10% to 40% of initially successful teeth needed re-treatment; cervical root resorption is a possible consequence, and is more frequent with the thermo-catalytic technique.
Mechanistic cautionary findings: direct contact with hydrogen peroxide produced genotoxic effects in bacteria and in cultured cells, an effect reduced or abolished in the presence of metabolising enzymes; several tumour-promotion studies, including the hamster cheek pouch model, indicated peroxide might act as a promoter; and in animals, multiple exposures produced local gastric mucosal effects, decreased food consumption, reduced weight gain and blood chemistry changes.
Their recommendation, stated as a threshold: avoid using concentrations higher than 10% carbamide peroxide for external bleaching. Their risk assessment found a sufficient safety margin was not reached in specific regimens — one arch at 35% carbamide peroxide, several applications per day at 22%, or both arches at once at 22% (Dahl et al., Crit Rev Oral Biol Med 2003-01-01, PMID 12907697).

How to weigh this without panic or dismissal. The animal and cell findings are not human epidemiology, and the review’s own framing is about safety margins in specific high-dose regimens rather than a claim that whitening causes disease. At the same time, “it is only peroxide, it is harmless” is not what the literature says either — it says: the effect you want is a function of concentration and time; the sensitivity is common; the concentration threshold they advise is around 10% carbamide peroxide; and supervision is what the mechanism review identifies as the difference between a good and a bad outcome (Joiner et al., J Dent 2006-08-01, PMID 16569473) (Kwon et al., J Esthet Restor Dent 2015-09-01, PMID 25969131) (Dahl et al., Crit Rev Oral Biol Med 2003-01-01, PMID 12907697).

What whitening cannot fix

Bleaching acts on natural tooth structure. It does not change the colour of composite, crowns, veneers or existing fillings — which is why whitening immediately before replacing a front-tooth composite produces a mismatch, and why the standard sequence is whiten first, wait, then match and place. Restorations that must be replaced after whitening are a foreseeable cost, and the reason a “one-week wedding kit” before an event is a bad plan if your front teeth are restored.

Mouthwash-grade causes of staining (tobacco, coffee, tea, chlorhexidine) will re-stain freshly whitened enamel faster than they stained it before, because you have just removed the baseline of colour you were measuring against. Expect maintenance; do not expect the shade in the brochure to be permanent.

Grey or brown discolouration from tetracycline, fluorosis, or a non-vital single tooth is a different problem with a different answer (microabrasion, veneers, internal bleaching of the single tooth with its own re-treatment and resorption caveats above) (Dahl et al., Crit Rev Oral Biol Med 2003-01-01, PMID 12907697).

Before you buy anything: the checklist

Get cavities, leaking margins and gum disease treated first. Peroxide on an open cavity is not merely uncomfortable — it is the classic way to convert a simple filling into a root canal conversation.
Ask whether you have exposed root surfaces or recession. If yes, expect more sensitivity and ask about a desensitising protocol; fluoride toothpaste and, where available, professionally applied fluoride varnish are the usual levers.
Check whether your front teeth are restored. Whitening before replacing a composite is the sequence error that generates most “my tooth turned two colours” complaints.
Ask for the actual concentration on the label. Carbamide peroxide strength and hydrogen peroxide strength are not the same number; a “35%” product is not comparable to a “35%” one from another shelf.
Use a tray that fits. Overflow is where gingival irritation and the “white burned gums” photos come from, and poorly fitting strip systems leave the whitening uneven at the gumline.
Keep a record of your starting shade (a shade tab photo in daylight) so you can judge the result instead of the marketing.
Do not whiten during pregnancy or breastfeeding as a cosmetic routine: the reviewed literature does not establish safety in these groups, which is a reason to postpone, not a claim of harm.

Evidence at a glance

Source	What it reviewed	Statements as published	What to hold lightly
Joiner, J Dent 2006 (Joiner et al., J Dent 2006-08-01, PMID 16569473)	Literature on external bleaching of vital teeth: mechanisms, measurement methods, efficacy factors	Efficacy driven by concentration and time; lower concentrations can approach higher ones with longer wear; light-activation evidence “limited and conflicting”; stain type, initial colour and age matter	A narrative review, not a pooled meta-analysis
Kwon & Wertz, J Esthet Restor Dent 2015 (Kwon et al., J Esthet Restor Dent 2015-09-01, PMID 25969131)	Diffusion of agents, interaction with stains, surface change, ADA Council on Scientific Affairs position	Whitening is a dynamic process with micromorphological and optical changes; professional supervision is “imperative” for a successful and safe outcome	Mechanistic synthesis; effect sizes are not its subject
Dahl & Pallesen, Crit Rev Oral Biol Med 2003 (Dahl et al., Crit Rev Oral Biol Med 2003-01-01, PMID 12907697)	Biological aspects: short- and long-term effects, animal and in vitro data, risk assessment	Sensitivity in 15–78% of patients; internal bleaching re-treatment 10–40% over 1–8 years; avoid >10% carbamide peroxide for external bleaching; safety margin not reached in specified high-dose regimens	The preclinical findings are not human outcome data; the review itself is 2003, before recent formulation changes

What the evidence does not support

That a stronger gel is simply “better”. It is faster, with more irritation, and the lower-concentration route reaches comparable results with time (Joiner et al., J Dent 2006-08-01, PMID 16569473).
That lamps and lasers are proven boosters: the comparative clinical evidence is described as limited and conflicting (Joiner et al., J Dent 2006-08-01, PMID 16569473).
That whitening is free of meaningful side effects: sensitivity in 15–78% is a common event, and data on other adverse effects were described as lacking (Dahl et al., Crit Rev Oral Biol Med 2003-01-01, PMID 12907697).
That it can whiten crowns and composite. It cannot; they must be replaced and re-matched.
That any of this supports whitening “to fix” a single dark tooth caused by a dead pulp — that is a diagnosis and treatment question, not a cosmetics one.

Frequently asked questions

How long does whitening last? It is not permanent: re-staining is continuous and depends on diet, tobacco and hygiene. Plan for maintenance rather than a one-time fix; the reviews describe the process and its limits, not a guaranteed duration.

Does it damage enamel? The mechanism review describes changes in surface micromorphology and optical properties as part of how the process works (Kwon et al., J Esthet Restor Dent 2015-09-01, PMID 25969131); the 2003 review’s concern is dose-dependent irritation and insufficient safety margin at specific high concentrations (Dahl et al., Crit Rev Oral Biol Med 2003-01-01, PMID 12907697). Used at recommended strengths on sound, examined teeth, the literature does not describe enamel destruction — and it also does not support unlimited “top-ups”.

Why do my teeth hurt after? That is the common pathway: sensitivity in a wide share of patients (15–78%). It is usually transient. If pain is spontaneous, lingers at night, or one tooth is much worse than the rest, that is not normal whitening sensitivity and needs an examination.

Strip trays or dentist? Strips are lower concentration, fixed-shape, and cannot respect your gingival contour; custom trays with supervision control the dose, the contact time and — most importantly — the pre-whitening examination. The mechanism review’s point about supervision is exactly this (Kwon et al., J Esthet Restor Dent 2015-09-01, PMID 25969131).

Can I use charcoal powder or “acid-free natural” kits? Abrasives do not change chromophores inside the tooth; they remove surface stain and can remove structure. The reviewed mechanism is chemical oxidation, not polishing (Joiner et al., J Dent 2006-08-01, PMID 16569473).

I have crowns — will whitening match them? No. Whiten first, then re-evaluate and replace restorations to match the new shade.

Is 35% “too much”? The 2003 risk assessment singled out high-concentration, high-frequency regimens as the cases where the safety margin was not adequate, and recommended staying at or below 10% carbamide peroxide for external bleaching (Dahl et al., Crit Rev Oral Biol Med 2003-01-01, PMID 12907697). If your dentist uses a higher in-office concentration, that is a supervision-and-barrier context, not the same thing as doing it yourself for an hour a day.

Do whitening toothpastes work? They are, in the main, stain-removal products. They cannot replicate peroxide diffusion into dentin; at best they restore a bit of surface brightness (Joiner et al., J Dent 2006-08-01, PMID 16569473).

How soon before an event? Whiten with at least a couple of weeks’ margin, and never schedule new front-tooth bonding or veneers in the same fortnight: shade matching is unstable right after bleaching.

Do I need anything for the sensitivity? Ask your dentist about a desensitising plan (potassium-nitrate toothpaste, spacing applications, professionally applied fluoride varnish) before starting, rather than after the third painful day.

Glossary

carbamide peroxide vs hydrogen peroxide ↔ the former decomposes into the latter; gels are labelled differently, and equal-looking numbers are not equal strengths.
chromophores ↔ the coloured molecules inside enamel and dentin that peroxide oxidises; this is why whitening lightens from inside, not by polishing (Joiner et al., J Dent 2006-08-01, PMID 16569473).
external vs internal bleaching ↔ on vital teeth from outside, versus inside a root-filled tooth — the latter carries documented re-treatment rates of 10–40% over 1–8 years and a resorption risk with thermo-catalytic techniques (Dahl et al., Crit Rev Oral Biol Med 2003-01-01, PMID 12907697).
shade tab / baseline photo ↔ your only defence against “it looks whiter in the shop mirror”: dehydration immediately after in-office treatment overstates the result for hours.
overfill / gingival blanching ↔ gel pushed onto soft tissue; the commonest avoidable cause of pain in home trays, and the reason tray fit matters.

How this page was built, and what it cannot tell you

Three reviews were used deliberately rather than a pile of product trials, because the question at issue — how whitening works, what determines the result, and where the safety margins sit — is best answered by synthesis. Bibliographic records were retrieved and verified programmatically from Europe PMC. Note their dates: the safety review is from 2003 and the efficacy review from 2006; formulations have changed since, which is a real limitation of relying on them, and one reason professional supervision is part of the recommendation rather than an upsell (Kwon et al., J Esthet Restor Dent 2015-09-01, PMID 25969131).

What it cannot tell you: your starting shade and therefore your ceiling; whether one of your teeth will hurt because of an unnoticed crack or cavity; whether your crowns and veneers are in the frame; and how much re-staining your own coffee habit produces. Those are answered by an examination with a shade tab, not by a review.

This article summarises published research for information only. It is not medical or dental advice and does not replace an examination by a licensed dentist. Do not whiten over untreated decay or gum disease, and stop and seek assessment if pain is severe, one-sided or persists after you stop.

Sources

Peer-reviewed evidence

Joiner A. The bleaching of teeth: a review of the literature. J Dent 2006-08-01;34(7):412-419. doi:10.1016/j.jdent.2006.02.002 · PMID 16569473 · cited by 479 (Europe PMC)
Kwon SR, Wertz PW. Review of the Mechanism of Tooth Whitening. J Esthet Restor Dent 2015-09-01;27(5):240-257. doi:10.1111/jerd.12152 · PMID 25969131 · cited by 306 (Europe PMC)
Dahl JE, Pallesen U. Tooth bleaching–a critical review of the biological aspects. Crit Rev Oral Biol Med 2003-01-01;14(4):292-304. doi:10.1177/154411130301400406 · PMID 12907697 · cited by 286 (Europe PMC)

Bibliographic records above were retrieved and verified programmatically from Europe PMC (authors, journal, volume, pages, DOI, PMID, citation count). Coverage, guideline and regulatory statements are quoted from the cited public documents with their dates; verify against the current version before relying on them. This article is not medical or dental advice.