Teeth Whitening: The Complete Guide (Trays and In-Chair)

Teeth whitening is one of the most requested aesthetic treatments in dental practice, and one of the most poorly understood by the general public. Between the cheap kits sold online, the marketing promises of a “Hollywood smile in an hour”, and the various preconceptions inherited from the dental chair, it is easy to get lost. Yet behind what looks like a simple procedure, whitening remains a regulated medical treatment that relies on active molecules, precise indications, and a rigorous protocol.

This guide details what an adult patient should know before considering whitening: why teeth become discoloured, what whitening can or cannot correct, and above all the two main pathways offered in dental practice, at-home whitening with custom trays and in-chair whitening. The aim is not to promise a result, but to give patients the means to choose a method suited to their situation, with full understanding.

1. Why teeth become discoloured

A tooth does not have a single colour. What we perceive as the shade of a tooth actually results from the combination of the enamel, which is translucent, and the underlying dentine, which is more yellow and more opaque. Over time, the enamel wears and thins slightly, and the dentine mineralises further and darkens. In other words, teeth that turn yellow are often a normal consequence of ageing, not a sign of poor hygiene.

Practitioners distinguish two main types of discolouration.

Extrinsic discolouration

These are the superficial stains, deposited on the enamel by dietary or environmental pigments. They are largely dominated by:

• Coffee, black tea, red wine, dark sodas.
• Strong spices such as curry or turmeric.
• Tobacco, whose nicotine and tars leave particularly stubborn deposits.
• Certain chlorhexidine mouthwashes used over long periods.

These stains settle mainly on tartar and plaque. Professional scaling, possibly supplemented by air polishing, removes them in the vast majority of cases. This is therefore not yet whitening in the strict sense.

Intrinsic discolouration

These are deep stains, lodged within the dentine or enamel, that scaling does not remove. They account for most genuine whitening requests. Causes include:

• Natural ageing, by far the most common cause.
• Prolonged consumption of dietary pigments that have penetrated the dental structure.
• An old trauma to a tooth, which can darken it after loss of vitality.
• The use of tetracyclines during tooth formation (before age 8).
• Fluorosis linked to excessive fluoride exposure during childhood.
• Certain dental pathologies or enamel malformations.

The distinction between extrinsic and intrinsic discolouration is decisive: only a clinical examination can determine which is involved, and therefore whether whitening is appropriate, at what intensity, and with what protocol.

2. What whitening is not

Before discussing methods, it is useful to clarify a few common confusions.

Whitening is not scaling

Scaling removes plaque, tartar, and superficial stains. The teeth recover their natural shade, which often gives an impression of lightening. But the intrinsic colour of the tooth is not changed.

Whitening, on the other hand, acts chemically on the coloured molecules inside the dentine. It is a real shade change, not a simple cleaning.

Many patients who think they need whitening are actually candidates for a thorough scaling, possibly followed by air polishing. A prior examination avoids treating a non-existent problem.

Whitening is not whitening toothpaste

Whitening toothpastes sold in supermarkets only act on the surface of the enamel. They contain either abrasives, which polish the tooth (with a risk of long-term wear if highly abrasive), or small quantities of lightening agents that are insufficient for a real change of shade.

They can help maintain a result after whitening, provided they are chosen carefully. They do not replace a treatment.

Whitening is not a trivial procedure

Whitening is a regulated medical treatment. It relies on active molecules subject to regulation, and requires a prior examination to:

• Detect cavities, cracks, and faulty old restorations, which can let the product penetrate into the pulp.
• Check the health of the gums, to avoid any irritation.
• Anticipate cases where certain teeth (crowns, veneers, composites) will not whiten.
• Adapt the method and concentration to the patient’s profile.

Whitening carried out without an examination, particularly via anonymous online kits, exposes the patient to avoidable complications.

3. The chemical principle common to both methods

Whether performed at home or in-chair, whitening relies on the same chemical principle: an oxidising molecule capable of crossing the enamel and reaching the dentine, where it breaks down the chromophores responsible for the discolouration.

Two molecules are used.

Hydrogen peroxide

This is the fastest and most powerful active molecule. It is released immediately on contact with the tooth. It is the form used in in-chair whitening, at varying concentrations depending on the protocol.

Carbamide peroxide

This is a more stable complex which, on contact with saliva, slowly degrades into hydrogen peroxide and urea. Its gradual release makes it better tolerated over long periods, which makes it the molecule of choice for at-home tray whitening. Carbamide peroxide at 10% releases approximately 3.5% of active hydrogen peroxide.

In both cases, the chemical reaction is the same: oxidation of the chromophores, lightening of the shade. The difference between methods comes down to concentration, application time, and context (clinic or home).

Since 2012, European regulations have strictly governed professional whitening products: hydrogen peroxide concentrations above 0.1% can only be used by, or under the responsibility of, a dentist, with a mandatory first examination and first application in the dental practice before any continuation at home. Switzerland applies a comparable framework.

4. At-home whitening with custom trays (ambulatory)

This is the first main treatment pathway. It involves wearing, at home, soft custom trays containing a whitening gel, over several days or weeks.

The protocol

Treatment always begins at the practice.

1. Clinical examination and, if needed, a radiographic assessment. The practitioner evaluates the condition of the enamel, the gums, and any existing restorations, and discusses a realistic shade goal with the patient.
2. Prior scaling if necessary, to start with clean surfaces.
3. Taking impressions of the dental arches, increasingly often digital, to design perfectly fitted trays.
4. Custom fabrication of the trays, thin and soft, with reservoirs that hold the gel against the visible surfaces of the teeth without overflowing onto the gums.
5. Handover of the trays and the gel, with a demonstration of the protocol: dose, positioning, duration, frequency, precautions.

How it works at home

The patient applies a small quantity of gel into each tooth slot, inserts the trays, and keeps them in:

• Either for a few hours per day, during the day, with moderate-concentration gels.
• Or overnight, with gels specifically designed for night-time wear.

Treatment generally lasts two to four weeks. An interim review at the practice allows for adjustments if necessary (reduced daily wear time in case of sensitivity, change of concentration, tray adjustment).

Advantages

• Gradual progression: the result is achieved in stages, which allows the patient to stop once the desired shade is reached, even mid-treatment.
• Comfort: the concentrations used at home are lower than those used in-chair, which limits sensitivity.
• Maintenance: the trays are kept. They can be used for future top-ups, at spaced intervals, to maintain the result over time.
• Cost: lower than in-chair whitening, especially relative to the duration of the result.

Limitations

• Discipline required: the patient must follow the daily protocol for several weeks.
• Tooth sensitivity: relatively common, most often transient, it may require a pause or a desensitising toothpaste.
• Slower result than in-chair whitening; not well suited if a short-term event is the goal.
• Gum irritation is possible if the trays fit poorly or if the gel is overdosed.

Over-the-counter kits

Whitening kits sold without going through a dentist fall into two categories:

• Thermoformable tray kits: the tray is not custom-made, it is roughly adapted by heating. It fits the arches less well, which lets the gel overflow onto the gum and reduces effectiveness.
• Strip or brush kits: easy to use, but generally with concentrations limited by consumer regulations, with a modest result that is not tailored to the patient’s profile.

These kits are not necessarily dangerous if they comply with legal concentrations, but by definition they bypass the prior diagnosis. Used on undiagnosed cavities, faulty restorations, or non-vital teeth, they can worsen existing problems or produce an uneven result.

5. In-chair whitening (in the practice)

This is the second main pathway, carried out entirely at the practice by the practitioner.

The protocol

1. Complete clinical examination and possible initial shade reading using a shade guide, to objectively record the starting point.
2. Prior scaling if necessary.
3. Protection of the soft tissues: lip retractor, gauze, and above all application of a light-cured resin dam. This band fits precisely against the gum line to isolate the gum from the product, an essential step to avoid superficial burns.
4. Application of the whitening gel, at a higher concentration than those used at home (hydrogen peroxide, sometimes at 25-40% depending on the product).
5. Possible activation by an LED lamp or laser. The actual benefit of this activation is debated in the literature: some products derive a benefit from it, others do not differ significantly from simple application. Modern “self-activating” products do not require it.
6. Application time, then rinsing. The cycle is generally repeated two or three times in the same session.
7. Removal of the dam, careful rinsing, application of a soothing fluoride gel. Final shade reading.

How it works

A complete session generally lasts between 60 and 90 minutes. In most cases, a single session is enough to obtain a noticeable shade improvement. Some situations (marked intrinsic discolouration, high expectations) may justify a second session a few days or weeks later.

Advantages

• Speed: the result is visible immediately at the end of the session.
• Supervision: the treatment is carried out entirely under the practitioner’s control, which limits handling errors.
• No compliance required: no trays to wear at home, no daily management.
• Well suited to events: weddings, professional interviews, photo shoots, where a short-term result is sought.

Limitations

• Post-operative sensitivity is more pronounced than with the ambulatory method, due to the higher concentration used. It typically lasts 24 to 72 hours, rarely longer. A preventive or post-operative desensitising treatment is commonly offered.
• Higher cost, linked to chair time and equipment.
• More marked rebound: the final shade, measured immediately after the session, partly reflects a transient dehydration of the enamel; it stabilises within 24 to 48 hours at a slightly less light, but lasting, shade.
• No autonomous maintenance without add-ons (maintenance trays).

One method, two common variants

Many modern protocols are neither strictly in-chair nor strictly at-home. The two approaches are combined:

• In-chair then at-home: a session at the practice to quickly achieve a shade improvement, followed by two to three weeks of at-home trays to consolidate and even out the result.
• At-home then in-chair top-up: several weeks of ambulatory treatment, then a short session at the practice to reach the target shade and finalise.

This mixed approach often gives the most stable results over time.

6. Which to choose: comparison

The choice between the two methods is not based on preference, but on a combination of factors.

Criterion	At-home whitening	In-chair whitening
Treatment duration	2 to 4 weeks	1 session (sometimes 2)
Sensitivity	Moderate, spread out	More pronounced, brief
Cost	Lower	Higher
Compliance required	High	Low
Reversibility upon stopping	Yes, at any time	No, once the session is complete
Suited to short-term events	No	Yes
Autonomous maintenance	Yes, with the trays	No without add-ons

In summary: at-home whitening is better suited to organised patients, who seek a gradual, more comfortable, and lasting result with maintenance. In-chair whitening is better suited to patients in a hurry, who want a quick, supervised result, and who accept more pronounced sensitivity. For most indications, the two methods lead to a comparable result in the medium term, and are often offered in combination.

The final choice is made with the dentist, after examination, and according to:

• The cause of the discolouration (extrinsic, intrinsic, ageing, pathology).
• The condition of the enamel and gums.
• The presence of restorations on the visible teeth.
• The realistic aesthetic goal, given the starting shade.
• The patient’s sensitivity profile.

7. Longevity of the result and maintenance

Whitening is not permanent. The teeth continue to undergo the same stresses as before, and the shade gradually fades.

On average, a result is maintained for between one and three years, with significant individual variations. Factors that prolong the result:

• Rigorous hygiene: twice-daily brushing, daily interdental cleaning, regular scalings.
• Moderate consumption of coffee, tea, red wine, pigment-rich sauces. There is no need to give them up, but rinsing or brushing within a reasonable time afterwards helps.
• Stopping smoking, which remains by far the most degrading factor in the long term.
• Use of a suitable toothpaste, non-abrasive, possibly specialised.

Maintenance is generally carried out at the practice: occasional top-ups using at-home trays (once or twice a year depending on the case), or a short in-chair session if the shade rebound is marked. A patient who has kept their custom trays can refresh the shade for a very limited cost, without starting from scratch.

8. Safety, side effects and contraindications

Whitening, when properly supervised, is a procedure with well-documented safety. But it is not without effects.

Tooth sensitivity

This is the most common side effect. It manifests as a sensation to heat, cold, or sometimes air, during and for a few days after the treatment. Several factors modulate it: product concentration, treatment duration, initial condition of the enamel, gum recession exposing the root.

It is almost always reversible within 24 to 72 hours, sometimes a little longer. Strategies for prevention and management:

• Desensitising toothpaste with potassium nitrate, in the days before and after the treatment.
• Application of fluoride or CPP-ACP-based gel.
• Reduced tray wear time.
• Spacing out of in-chair sessions.

Gum irritation

This occurs mainly when the gel comes into prolonged contact with the gum. In-chair, the resin dam prevents it. At home, well-fitted trays and a correct gel dose are enough to avoid it in the vast majority of cases. Irritation heals spontaneously within a few days.

Long-term effects

At recommended doses and frequencies, whitening does not weaken the enamel in any clinically significant way. Long-term follow-up studies show no increase in caries risk or accelerated wear.

On the other hand, overuse (a patient who repeats whitening outside any follow-up, exceeds recommended durations, or uses products at illegal concentrations) can lead to superficial demineralisation and lasting sensitivity. Hence the importance of professional supervision.

Contraindications

Whitening is not recommended, or must be deferred, in several situations:

• Pregnancy and breastfeeding: as a precaution, even though the literature does not demonstrate a risk.
• Children and adolescents until enamel formation is complete, except for specific cases handled individually.
• Active cavities or untreated faulty restorations.
• Visible cracks or fractures in the enamel.
• Active gingivitis or periodontitis, to be stabilised before any whitening.
• Significant gum recession exposing the root, an area very sensitive to peroxide.
• Known allergy to peroxide or to the components of the gel.

It is precisely the prior examination that allows these situations to be identified and a suitable solution proposed.

9. Whitening and dental restorations

A point often overlooked before treatment: prosthetic restorations do not whiten. Crowns, ceramic veneers, composites, bridges and inlays keep their original shade. Whitening on a smile that includes visible restorations can therefore create a shade mismatch between the lightened natural teeth and the unchanged prosthetics.

Two classic strategies take this into account:

• Whiten first, wait for the shade to stabilise (two to four weeks), then redo the anterior restorations in the new shade. This is the cleanest approach when a change of prosthetics is planned anyway.
• Accept a possible mismatch if the area concerned is not very visible, and discuss the expected result with the practitioner before starting.

A specific case: the non-vital tooth that has darkened after endodontic treatment. It is not treated by external whitening, but by internal whitening, a specific technique in which the product is placed inside the tooth after re-treatment and isolation. It is a separate procedure, performed at the practice, with its own indications.

10. Myths and frequently asked questions

“The higher the concentration, the more effective the whitening”

Partly false. Beyond a certain threshold, increasing the concentration mainly produces more sensitivity, not a significant gain in shade. The best results come from a suitable product, applied under good conditions and for the right duration, not from the strongest product.

“Teeth become as white as paper”

False. A healthy tooth has natural translucency. A harmonious result remains within the range of natural shades, simply lighter than the starting point. A “bright blue-white” shade, as sometimes seen in retouched images, is not a physiological goal.

“It damages the teeth”

False, within the framework of a supervised protocol. Long-term follow-up studies show no significant alteration of the enamel or dentine under recommended conditions of use.

What can really damage the teeth: products at illegal concentrations, application without prior diagnosis, abusive frequency, or daily use of highly abrasive “whitening” toothpastes.

“I can do it all myself without going to a dentist”

Technically possible, regulated by law. Over-the-counter products are at limited concentrations, with a modest result. Above all, no diagnosis is made. A kit used on an undetected cavity or a non-vital tooth can worsen the situation. A prior consultation, even a short one, is the right precaution.

“Bicarbonate, activated charcoal, coconut oil whiten the teeth”

No. These substances have no demonstrated lightening effect in the chemical sense of the term. Bicarbonate and charcoal, which are highly abrasive, can superficially polish the enamel and give it a cleaner appearance in the short term, at the cost of increased wear in the long term. Coconut oil has no chemical action on shade.

11. Conclusion and when to consult

Teeth whitening is a treatment that is both effective and regulated, provided it is chosen for the right reason, on a healthy mouth, and with a suitable method. The two main pathways, at-home whitening with custom trays and in-chair whitening, address different profiles and are often complementary. There is no universally “better” method: there is a method suited to each patient.

Before considering treatment, here are some indications for consulting:

• A shade considered unsightly by the patient, with no improvement from usual hygiene care.
• An isolated tooth darker than the others, a possible sign of loss of vitality.
• A broader aesthetic project (prosthetics, veneers) whose target shade depends on prior whitening.
• A short-term event motivating a quick result.

In every case, an examination distinguishes what comes under simple scaling, whitening, restoration, or a combination of the three. The aim is not to have the whitest possible teeth, but a harmonious, healthy smile that lasts over time.