The active ingredients in teeth whitening gels are hydrogen peroxide and carbamide peroxide. Hydrogen peroxide diffuses into enamel and dentine and breaks down colour molecules by oxidation. Carbamide peroxide breaks down slowly and releases hydrogen peroxide equal to about a third of its weight. Over-the-counter products are limited to 0.1%; products up to 6% are used only under a dentist's supervision and only in over-18s.
Which active ingredients do whitening gels contain?
The most commonly used active ingredients in teeth whitening gels are hydrogen peroxide and carbamide peroxide. Both ultimately deliver hydrogen peroxide to the tooth; the difference between them is how quickly it is released.
As well as the active ingredient, gels contain other components. These affect the gel's consistency, how well it adheres to the tooth and the risk of sensitivity.
- Hydrogen peroxide: Acts directly; usually used in in-office gels applied at the clinic.
- Carbamide peroxide: Breaks down slowly to release hydrogen peroxide; mostly used in take-home tray whitening.
- Sodium perborate: An agent that can be used for internal whitening of root-filled teeth.
- Additional ingredients: Components aimed at reducing sensitivity, such as potassium nitrate and fluoride, as well as thickeners and water.
How does hydrogen peroxide whiten teeth?
Hydrogen peroxide diffuses into enamel and dentine and breaks down colour molecules by oxidation. Large, dark molecules are split into smaller, lighter fragments, which is why the tooth looks lighter.
At a microscopic level, enamel is porous. Peroxide passes through these pores and reaches the dentine. Because tooth colour is largely determined by the dentine, much of the effect takes place there.
This process does not break down the tooth's structure or dissolve enamel. However, the teeth lose some water during treatment, so straight afterwards they may look whiter than they really are. Within a few days the teeth rehydrate and the shade settles.
What is carbamide peroxide and how does it work?
Carbamide peroxide is a compound of hydrogen peroxide and urea. In the mouth and on the tooth it breaks down to release hydrogen peroxide equal to about a third of its weight.
For example, 10% carbamide peroxide corresponds to about 3.5% hydrogen peroxide. Because it breaks down more slowly, its effect is spread over several hours. This makes carbamide peroxide suitable for take-home teeth whitening with trays worn overnight or for long periods.
The urea released as it breaks down helps balance the acidity in the mouth. This can help make treatment more comfortable.
The difference between hydrogen peroxide and carbamide peroxide
Hydrogen peroxide acts quickly and directly; carbamide peroxide acts slowly and over a longer time. The table below compares the two objectively.
| Feature | Hydrogen peroxide | Carbamide peroxide |
|---|---|---|
| Speed of action | Fast | Slow, spread over hours |
| Hydrogen peroxide equivalent | Itself | About a third of its weight |
| Typical use | In-office treatment at the clinic | Take-home treatment with a custom tray |
| Treatment time | About 45–90 minutes per session | Usually daily for 1–3 weeks |
| Sensitivity | May be more noticeable over a short time | Usually develops more slowly |
| Supervision | By the dentist, in the clinic | First application at the dentist, then at home |
When comparing concentrations, always look at the hydrogen peroxide equivalent. For example, 16% carbamide peroxide is roughly equivalent to 5–6% hydrogen peroxide.
Legal limits: 0.1%, 6% and the under-18 rule
Under Turkish and EU cosmetics rules, there are two main limits for hydrogen peroxide (the amount contained or released) in teeth whitening products.
| Hydrogen peroxide level | Who can use it? | Conditions |
|---|---|---|
| 0.1% or less | Sold over the counter | Pharmacy, supermarket and online products |
| Above 0.1%, up to 6% | Sold only to dentists | First application by the dentist; not used in under-18s |
| Above 6% | Not permitted as a cosmetic product | Products exceeding these limits should be avoided |
These rules exist to prevent high-concentration products being used without an examination and dental supervision. We explain why over-the-counter products have a limited effect on our teeth whitening at home page. For details of the age limit, see our what age can you have teeth whitening page.
Does light or laser boost the gel's effect?
Lights and lasers are light sources used to activate the gel; what actually whitens is the peroxide in the gel. Any additional contribution of light to the result is scientifically debated.
In Türkiye the term “laser whitening” is widely used. The device used at our clinic is a blue-violet LED light. Some studies suggest light may speed up the process, while others have found no clear difference.
So when considering light-activated whitening, it makes more sense to focus on the gel concentration, treatment time and gum protection. For details, read our light-activated teeth whitening page.
Are peroxide gels safe?
Peroxide gels used within legal limits, after an examination and under dental supervision, are generally considered safe. Side effects are mostly temporary.
- Sensitivity: The most common side effect; it usually passes within 24–72 hours.
- Gum irritation: If the gel touches the gums, a whitish, temporary irritation may appear. In the clinic this risk is reduced with a protective gum barrier.
- Enamel surface: At appropriate concentrations, no permanent damage to enamel is expected; temporary surface changes are restored by saliva.
- Restorations: Fillings and crowns do not whiten; a colour mismatch may result.
- Overuse: With more frequent or longer use than recommended, teeth may look grey or translucent and sensitivity increases.
Whitening is not recommended during pregnancy, while breastfeeding or under the age of 18. Take care not to swallow the gel and wipe away any gel that overflows from the tray.
Peroxide-free whitening products
Some peroxide-free products are also sold as “whitening”; however, evidence that they lighten the tooth's own colour is generally limited.
These products may contain other oxidising agents, enzymes or stain-removing components. Some can reduce surface stains; others make teeth look temporarily less yellow using optical pigments. Acidic ones can erode enamel.
When assessing a product, check its active ingredient, concentration and instructions for use. If the label does not clearly state the ingredients and concentration, it is safer not to use it. If you are unsure, you can ask your dentist at your examination; the option that suits you is decided together, based on your existing fillings, gums and sensitivity.
Frequently asked questions
How much hydrogen peroxide is 10% carbamide peroxide equal to?
It is equal to about 3.5% hydrogen peroxide. Carbamide peroxide releases hydrogen peroxide equal to about a third of its weight.
Can you whiten teeth with hydrogen peroxide from the chemist?
This is not recommended. These products are not made for teeth whitening; uncontrolled use can cause gum irritation and sensitivity. Whitening should be done with products made for the purpose and under dental supervision.
Does hydrogen peroxide dissolve enamel?
Used within legal limits and correctly, it does not dissolve enamel. It works by oxidising colour molecules; temporary surface changes are restored by saliva.
Which active ingredient causes less sensitivity?
Because carbamide peroxide acts slowly, sensitivity usually develops more slowly. However, this varies from person to person and depends on concentration and duration.
What happens if whitening gel is swallowed?
Small amounts usually cause no serious problem, but may upset the stomach. Wipe away gel that overflows from the tray and do not exceed the recommended amount; if you have symptoms, consult your dentist.
Related pages
Light-Activated Teeth Whitening (In-Office)A single-appointment treatment carried out in the clinic under a dentist's supervision, using blue-violet light, whitening gel and a gum barrier.
Take-Home Teeth WhiteningControlled whitening done at home on your own schedule, with a custom tray and gel supplied by your dentist.Teeth Whitening at Home: Do Over-the-Counter Products Work?Find out what strips, pens, toothpastes and LED kits really do, their risks, and how they differ from dentist-supervised take-home whitening.
At What Age Can Teeth Be Whitened?The reason for the 18-year age limit, why young people should wait and how whitening is planned in later life.
Sensitivity After Teeth WhiteningWhy teeth feel sensitive after whitening, how long it lasts and what you can do to ease it.Suitability is decided at an examination. Fill in the form or call us to book an appointment.
Last updated: 2026-11-03 · This page is for general information only and does not replace an examination or diagnosis.