What peroxide actually does inside a tooth
The fear behind this question is usually a specific image: acid eating away the surface of the tooth, leaving it thin and brittle. That is not what happens, and it is worth understanding why.
The colour you dislike comes from large organic molecules — chromophores — that have accumulated in the tooth over years from food, drink, tobacco, injury or the natural ageing of dentine. These molecules are big enough to absorb visible light, which is what makes them look yellow or brown.
Hydrogen peroxide is a small, unstable molecule. Applied to a tooth, it diffuses through the enamel — which is porous at a microscopic level, whatever it looks like — and into the dentine, releasing reactive oxygen species along the way. Those break the long pigment molecules into smaller fragments. Smaller fragments absorb less visible light. The tooth looks lighter. That is the entire mechanism: not removal of tooth substance, but chemical alteration of the pigment trapped inside it.
What happens to the enamel surface? Studies of whitened enamel do describe changes: a temporary drop in surface hardness, slightly increased porosity, some loss of minerals from the outermost layer during the treatment. Two things put this in perspective. It is measured in microns and it is reversible — saliva delivers calcium and phosphate back to the surface within hours to days, and fluoride accelerates it. This is one reason professional protocols include fluoride application afterwards and advise against acidic food and drink for a day or so, and one reason we would rather you did not run a peroxide treatment on your own timetable with no idea what to do afterwards.
Does it harm the nerve? During whitening, traces of peroxide reach the pulp and provoke a reversible inflammatory response — that is what you feel as sensitivity. In healthy teeth this settles. Lasting damage to the pulp is not a recognised outcome of supervised whitening at accepted concentrations. Where a tooth has a deep cavity, a crack or a failing filling, the situation is different, which is the practical reason for examining first rather than a matter of formality.
Where the actual risks are
Being honest about a treatment means naming its problems, not denying them.
Tooth sensitivity. The most common side effect by a distance. Short, sharp reactions to cold during treatment and for a day or two afterwards, caused by fluid movement in the dentine tubules. It settles for the great majority of patients, and the risk is reduced by lower concentrations, shorter contact times and a desensitising product used beforehand. If your teeth already react to cold, Can you whiten sensitive teeth? covers what to do about it.
Gum and mucosal irritation. Peroxide on soft tissue causes a whitish, tender patch — a chemical irritation that typically resolves within a day or two. In the practice this is prevented by a protective barrier along the gum line. In home whitening it is prevented by a tray that actually fits, which is exactly what an off-the-shelf, one-size tray does not.
Whitening a mouth that has not been examined. This is the risk that matters most and gets discussed least. Gel travelling into an untreated cavity, under a leaking filling margin, or through an unnoticed crack reaches tissue it should not reach, and the result can be genuine pain rather than a twinge. An examination beforehand is not upselling; it is the difference between a cosmetic procedure and a gamble, and it is the first step of every whitening plan we make — see our Teeth whitening – service page (EN).
Restorations that do not change colour. Crowns (Kronen), veneers, white fillings and bridges keep their original shade. Whitening the natural teeth around them can therefore make a previously invisible restoration stand out. This is not damage, but it is a poor outcome, and it is avoidable with planning — see Whitening or veneers? How to decide.
Overuse. Whitening is not a daily cosmetic. Repeating it constantly in pursuit of a shade that is not achievable is the scenario in which cumulative surface effects stop being negligible, and it usually points to expectations that no treatment will satisfy. A sensible interval and an honest conversation about what your teeth can do prevent it.
Products from outside the EU and applications by people without dental training. Products ordered online from outside Europe may contain concentrations that are not permitted for self-application here, sold without the safeguards European rules attach to them. Applied without an examination, without gum protection and without anyone able to react if something goes wrong, the same chemistry that is safe in a dental setting becomes unpredictable.
Like a stain in fabric: the colour comes out, the fibres stay — with a tooth, the substance stays.
The rules in Germany: who may apply what
This surprises many people who move here, because their home country may regulate whitening quite differently — in some places beauty salons offer it, in others it is entirely a medical matter.
In the EU, whitening products are regulated as cosmetics under Regulation (EC) No 1223/2009, and the concentration of hydrogen peroxide (Wasserstoffperoxid) decides who may use them. Up to 0.1 % contained or released, a product may be sold freely to consumers — which is the whole reason drugstore strips and pens produce such modest results. Above 0.1 % and up to 6 %, products may be supplied only to dentists: the first application of each cycle is carried out by the dentist or under a dentist’s direct supervision, the patient must be examined for anything that would make the treatment inappropriate, and only then may the product be handed over to finish the cycle at home. The labelling of these products states that they are not to be used on anyone under 18. Concentrations above 6 % are not permitted as cosmetics in the EU at all.
Two consequences follow that are worth knowing:
Whitening for under-18s is not permitted with these products. Beyond the legal position there is a clinical reason: the pulp is larger and the enamel more permeable in young teeth, so sensitivity is greater.
The examination is part of the rule, not an add-on. Being told you need a check-up and possibly a cleaning before whitening is not a sales technique. It is how the treatment is meant to be delivered here.
If a whitening offer skips the examination, that is the point at which to ask questions — regardless of who is offering it or how convenient it looks.
Who should not whiten, or should wait
- Untreated decay, defective fillings, gum disease. Treat first, whiten afterwards. This is not negotiable, and the sequence protects you.
- Pregnancy and breastfeeding. No evidence of harm exists, but nor is there any reason to whiten during pregnancy — it is elective, it can wait, and gums are more reactive at that time anyway.
- Under 18. As above.
- Marked erosion or exposed tooth necks. Where enamel is thin or root surfaces are exposed, sensitivity is likely and the aesthetic result is often disappointing. The cause — reflux, diet, grinding — is dealt with first.
- Severe, generalised sensitivity that has not been investigated. Find out why before adding a chemical challenge.
- Allergy to peroxide preparations, which is rare but real.
- Very high expectations from a very dark starting point. Not a medical contraindication, but an honest one. Nobody can promise a specific shade, and deep intrinsic discolouration responds unpredictably. It is far better to hear this before treatment than to feel it afterwards.
- Heavy smokers who intend to carry on smoking during treatment. The result will not hold, and combining peroxide with tobacco during the treatment period is not sensible.
What's normal after whitening — and when to contact us
These are not disasters and most have a simple explanation, but they belong in a dental chair rather than in a search engine. Outside our opening hours the out-of-hours service (Notdienst) is there for genuine emergencies.
Special situations worth knowing about
Root-canal-treated teeth. A tooth that has darkened after a root canal treatment (Wurzelbehandlung) is a different case: it can often be lightened from the inside, with the whitening agent placed within the tooth. This is a dental procedure with its own rules and its own follow-up — and one that specifically requires a well-sealed root filling first.
Old fillings at the front. Composite that has discoloured will not lighten. The usual sequence is to whiten, wait about two weeks for the shade to settle, then renew the fillings in the new shade.
Grinding and clenching. Bruxismus wears the enamel at the biting edges and often produces sensitivity of its own. Where it is present, a night guard (Knirscherschiene) generally belongs in the plan before cosmetics.
Dental anxiety. Whitening is one of the few dental appointments with no drill, no needle and no noise. For anxious patients it can be a reasonable re-entry point, and we can support the appointment with sedation or laughing gas (Lachgas) if that is what makes it possible.
If you are new to Germany or your German is limited. Two things regularly cause confusion. First, whitening here is a dental treatment governed by EU cosmetics rules, not a beauty-salon service — so expect an examination first. Second, no statutory fund pays for it, so the price you are quoted is the price, in writing, before anything starts. We treat patients in English; the practical background to the German system is in English-Speaking Dentist in NRW. For cosmetic treatment patients come to us from across NRW — Düsseldorf, Cologne and Bonn are within about half an hour — and from further afield in Germany, which matters when you want the risks explained in a language you can question them in.
Not in the way most people picture. Peroxide alters the pigment inside the tooth rather than dissolving the surface. Measurable changes to the outermost enamel layer during treatment are small and reverse over the following days through remineralisation from saliva and fluoride. Enamel damage in this context comes far more often from abrasive home methods and dietary acid than from supervised whitening.
Peroxide irritates soft tissue if it makes contact, producing a whitish, tender patch that usually settles in a day or two. That is why the gums are protected with a barrier in the practice and why trays for home use are custom-made rather than one-size. Where whitening is done without those safeguards, gum irritation is a common complaint.
Lasting pulp damage is not a recognised consequence of supervised whitening at permitted concentrations. What occurs is a reversible irritation, experienced as temporary sensitivity. The risk changes if peroxide reaches the pulp through an untreated cavity or a deep crack — the reason an examination comes first.
There is no fixed number, and the useful answer depends on your starting shade, the method and how your teeth react. What is not sensible is repeating treatments back to back in pursuit of a shade your teeth cannot reach. A top-up at sensible intervals, decided together with your dentist, is the reasonable pattern.
It is normally postponed. There is no evidence of harm to the baby, but whitening is entirely elective and there is nothing to be gained from doing it now rather than later. Gums are also more prone to inflammation during pregnancy, which makes the treatment less comfortable and the timing worse.
Used as directed they are not dangerous — their peroxide content is legally capped at a low level, which also explains their limited effect. The greater risk lies with high-concentration products ordered from outside the EU and used without an examination or gum protection. If a product promises dramatic results and is freely available, that combination is not legally possible here.
Whitening has a reputation for being risky that the evidence does not support — provided it is done properly. The chemistry alters pigment inside the tooth rather than removing tooth substance, the surface effects are small and reversible, and the well-documented side effects, sensitivity and gum irritation, are temporary and manageable. What genuinely causes trouble is the shortcut: no examination, untreated decay, unprotected gums, a tray that does not fit, an unregulated product and nobody to turn to if it hurts. The European rules on who may apply which concentration exist for exactly that reason. Have the mouth checked, have the treatment planned, and whitening is one of the gentlest things in dentistry.
Zahnarztpraxis Tsypin · Wuppertal. This article was medically reviewed and is provided for information; it does not replace an examination. Updated August 2026.
