How Teeth Whitening Actually Works: Peroxide, Oxidation and Colour Change
This page explains how teeth whitening works, for anyone who wants the chemistry. Research reviews describe peroxide passing through enamel into dentine and oxidising coloured molecules so they reflect less colour. Concentration and time shape how fast it works, and a 2026 laboratory study found whether colour change levelled off depended on the threshold. Peroxide whitening is chemical, not abrasive, although a 2015 review suggests peroxide also affects sound enamel.
5.0 from 550+ Google reviews
Open 8am to 8pm, seven days
Free parking at the door
CQC & GDC registered
No referral needed
Clinically reviewed by Dr Ibraheem Ijaz, dentist · GDC 301711
Medical disclaimer: This page is for general information only. It does not replace an examination or treatment by a dentist.
Dr Ibraheem Ijaz reviewed this page. He is principal dentist at Deepcar Dental Care, a General Dental Practitioner registered with the General Dental Council (GDC) under number 301711, and holds a BDS, BSc (Hons) and MSc (Leeds).
How does teeth whitening work?
Peroxide teeth whitening works by chemistry: the gel breaks down coloured molecules in the tooth so they reflect less colour. A 2006 review concluded that the literature suggests peroxide diffuses through enamel and oxidises coloured compounds, particularly in the dentine, the layer beneath the enamel.
A 2005 European Commission scientific committee opinion (SCCP) describes complex stain molecules being broken into simpler ones that reflect less light. A 2019 review notes that the overall kinetics of these reactions, how fast they run, are not known.
What does peroxide do to a stain molecule?
Peroxide oxidises stain molecules, changing the part of their structure that absorbs light. These coloured molecules are called chromophores, and a 2019 review explains that in teeth they typically reflect yellow or brownish colours.
Many chromophores, such as tannins from coffee, tea and red wine, contain double bonds. A 2014 review explains that hydrogen peroxide oxidises these double bonds, turning the chromophore into a lighter coloured compound, while metal-containing stains are much more difficult to bleach. The SCCP describes hydrogen peroxide as a strong oxidant that can form free radicals, highly reactive fragments. Stain causes are explained under intrinsic and extrinsic staining.
Carbamide peroxide versus hydrogen peroxide: what is the difference?
Carbamide peroxide is a carrier for hydrogen peroxide: in contact with water it breaks down into hydrogen peroxide and urea. So, the 2014 review notes, the chemistry of most tooth whitening is that of hydrogen peroxide.
The SCCP states that carbamide peroxide contains approximately 35% hydrogen peroxide, and cites a 15% carbamide peroxide gel as equivalent to 5.3% hydrogen peroxide. In a laboratory study on extracted teeth, it reports, carbamide peroxide appeared to penetrate less than an equivalent amount of hydrogen peroxide.
Gel strength in UK law
UK cosmetics law sets whitening limits by the hydrogen peroxide contained or released, including from carbamide peroxide. As the General Dental Council (GDC) sets out, products containing or releasing 0.1% to 6% hydrogen peroxide are sold only to dental practitioners. Their conditions of use include an appropriate clinical examination and no use on anyone under 18 except wholly to treat or prevent disease. Above 6% is prohibited for cosmetic use. Who should not whiten is covered in teeth whitening age limits and pregnancy and whitening with gum disease or decay.
How deep does whitening peroxide act?
Whitening peroxide does not stay on the surface: enamel is permeable to it, and it passes on into dentine and pulp. The SCCP reports laboratory research showing this transport. The page on enamel, dentine and pulp explains each layer.
The SCCP describes sensitivity as a common adverse effect, and studies of teeth whitened before a planned extraction found minor inflammation in the outer pulp, which it says seems to coincide with that sensitivity. It adds that, despite peroxide reaching the pulp, pulp damage from whitening is remarkably low. Talk to your dentist if you have any side effects that bother you, NHS guidance says.
Whitening placed inside a root-filled tooth is a separate treatment, described under internal teeth whitening.
To ask about teeth whitening, book online or call 0114 288 2121.
Why does contact time matter?
Contact time matters because concentration and time are the key factors in peroxide whitening efficacy, a 2006 review found. In general, higher concentrations work faster, but lower concentrations can approach their effect with longer treatment times.
Longer contact is not open-ended. The GDC says exposure should be limited so products are used only as intended in frequency and duration. The 2014 review found the degree of side effects, such as sensitivity and mild gum irritation, is directly related to peroxide concentration, treatment length and the rest of the product's make-up. Talk to your dentist if you have any side effects that bother you, NHS guidance says.
Does whitening remove enamel?
Peroxide whitening works by chemistry rather than by wearing enamel away, but research suggests it can still affect enamel. The 2019 review explains that abrasives can remove stains inside the tooth only by removing part of the enamel, while bleaching agents penetrate the enamel and act chemically.
A 2015 review of the mechanism suggests that peroxide's interaction is not limited to stain molecules: it also affects sound enamel and dentine, including microscopic surface changes. It concluded that supervision by dental professionals is critical to a successful and safe outcome. In laboratory studies, the 2014 review reports, aggressive whitening with high-concentration hydrogen peroxide in-surgery products caused enamel softening, surface roughness and greater susceptibility to demineralisation.
Yet in one such study, teeth bleached with a 10% carbamide peroxide home regimen were more prone to later demineralisation than teeth given a light-activated 35% hydrogen peroxide in-surgery treatment, above the 6% UK cosmetic limit. The evidence is set out under whether whitening damages enamel and why sensitivity happens.
What is the plateau effect in teeth whitening?
The plateau effect, often called the saturation point, describes colour change slowing and levelling off as bleaching continues. A 2026 laboratory study noted that, though widely discussed, it had not been objectively described before.
The 2026 study bleached 60 extracted molars over 8 cycles with a home protocol (10% carbamide peroxide) or an in-surgery protocol (35% hydrogen peroxide, above the 6% UK cosmetic limit). On a looser threshold, most teeth stabilised within the study, with no difference between protocols. On a stricter one, 13 of 30 home and 11 of 30 in-surgery teeth stabilised on one colour measure, similarly on two others, though all did on a fourth. The authors advised that decisions on bleaching time follow the visible response, not a fixed number of cycles.
Tray whitening at Deepcar Dental Care
At Deepcar Dental Care, our dentists provide Enlighten and Boutique whitening, both delivered as a take-home tray system. With both, trays are worn for around 6 hours overnight or around 1.5 hours during the day.
Frequently asked questions
Is teeth whitening gel the same as bleach?
Not household bleach. In dentistry, bleaching means breaking down stain molecules chemically, and most whitening gels do it with hydrogen peroxide or carbamide peroxide. A 2014 review notes that whitening toothpastes contain no sodium hypochlorite, household bleach's chemical, though some professional products do, acting on stains in much the same way.
Can whitening toothpaste work the same way as a peroxide gel?
Mostly by other means. UK rules limit toothpaste to 0.1% hydrogen peroxide, and a 2019 review questions whether that gives enough oxidising power, given peroxide's instability, short brushing time and dilution by saliva. Abrasives, which remove surface stain mechanically but can wear teeth, are toothpaste's main stain-removing ingredient. The page on professional whitening and shop-bought kits compared covers kits.
What happens to stain molecules after whitening?
Organic ones are changed chemically rather than scraped off. A 2019 review explains that peroxide oxidises organic stain molecules into colourless compounds, which are assumed to be washed away afterwards. Coloured metal ions, such as iron, are not oxidised, but are thought to be released once the organic molecules around them have been broken down.
Does whitening work the same way on everyone's teeth?
Not always. A 2006 review lists the type of stain, the starting tooth colour and a person's age among the factors that can influence the outcome. A 2019 review adds that developmental enamel marks, such as fluorosis, are not completely removed by bleaching, and that the way many whitening agents act inside the mouth is still unknown.
Does the effect of teeth whitening last?
Not indefinitely. NHS guidance says that if you follow your dentist's advice, the effect of teeth whitening may last for around 3 years. A 2019 review notes that coloured compounds can be deposited on teeth again after whitening, which some toothpaste ingredients are intended to prevent. The page on how long teeth whitening lasts covers this.
Book a consultation.
Online in under a minute, or call and speak to a real person.
You can call Deepcar Dental Care on 0114 288 2121 or email [email protected]. The page on teeth whitening explains treatment with a dentist.
For stain types and more whitening chemistry, see the options in this section.
334 Manchester Road
Deepcar, Sheffield S36 2RH
0114 288 2121
Contact us
Opening hours
Open 8am to 8pm, seven days
Free parking at the door