The short version, if you only read one thing
Whitening works by oxidising the organic compounds in enamel and dentin that cause discolouration, using carbamide peroxide or hydrogen peroxide as the active agent. Professional in-office whitening uses 25–40% hydrogen peroxide for 30–60 minutes per session and produces 3–8 shade improvements in one or two visits. Cost: $400–$1,000. Professional at-home trays use 10–22% carbamide peroxide in custom-fitted trays worn nightly for two to three weeks; results are slightly less dramatic but more controlled. Cost: $150–$600. Drugstore strips and gels use 3–10% peroxide and produce noticeable but limited results over four to eight weeks. Cost: $25–$80. The single biggest variable in whether whitening will work for you is not the product — it's the underlying colour of your teeth. Naturally yellow teeth respond well to all forms of bleaching; naturally grey teeth respond poorly to any of them; tetracycline-stained teeth often respond inadequately to bleaching alone and may need veneers for a meaningful colour change. Sensitivity is common during treatment, manageable with proper protocol, and resolves within days of finishing.
What bleaching agents actually do
The chemistry is straightforward. Teeth get discoloured over time because organic compounds — from food, drink, tobacco, and the gradual ageing of enamel and dentin themselves — accumulate within the porous structure of the tooth. Bleaching agents (hydrogen peroxide and carbamide peroxide, which breaks down into hydrogen peroxide on contact with tissue) release reactive oxygen molecules that oxidise these dark organic compounds into smaller, lighter molecules. The tooth itself is not damaged in a clinically meaningful way at the concentrations used in supervised dental whitening1; the structure stays intact while the colour shifts.
The active oxygen molecules also pass through enamel into the dentin underneath, which is where most of the perceived tooth colour actually comes from. This is why bleaching can change the apparent colour of a tooth even though the visible surface (enamel) is naturally translucent. The result is not painting the tooth white — it is lightening the colour of the material underneath.
Concentration matters substantially. A 35% hydrogen peroxide gel used in office whitening releases active molecules many times faster than a 10% carbamide peroxide gel used in at-home trays, which is why office whitening produces faster results per session. The trade-off is that higher concentration also increases the risk of sensitivity and gum irritation if the gel contacts soft tissue — which is why office whitening is done with the gums carefully isolated and the at-home version uses lower concentrations that can be safely worn over longer periods without supervision.
Why some teeth respond better than others
This is the variable that most patients don't know about going in, and the one that determines whether whitening is a good investment for any particular person.
Naturally yellow teeth respond well to all forms of bleaching. The dark organic compounds that cause the yellow tone are exactly what the peroxide molecules oxidise effectively. Most patients with predominantly yellow staining see substantial improvement from any reasonable whitening protocol.
Naturally grey teeth respond poorly. The grey tone comes from different underlying causes — sometimes from the dentin layer itself being thinner or more translucent, sometimes from internal staining the bleaching agents can't reach effectively. Patients with grey teeth often experience disappointing results regardless of which whitening product they use, and the honest answer in many cases is that veneers are the only path to a meaningfully different colour.
Tetracycline-stained teeth — the deep grey-brown banding from antibiotic exposure during tooth development — respond very poorly to bleaching alone. Extended bleaching protocols (six months or longer) can produce some improvement, but the result is often still visibly imperfect. For significant tetracycline staining, veneers are usually the more appropriate procedure.
Teeth with existing restorations — crowns, veneers, large fillings — do not change colour during whitening. The natural tooth structure lightens; the restorations stay the same shade. This can produce an awkward mismatch in the smile if the patient has visible restorations they hadn't considered. The order of operations matters: whitening first, then any restoration work to match the new shade.
Teeth darkened from root canal treatment are a separate problem. Internal bleaching (placing the gel inside the tooth) is sometimes effective for these specific cases but requires a different protocol than surface whitening.
50–65%
Approximate proportion of patients who experience some sensitivity during professional teeth whitening
4. The sensitivity is real but almost always temporary, resolving within days of completing or pausing treatment. It is well-managed with desensitising toothpaste, lower-concentration protocols, and periodic breaks if needed. The published evidence does not suggest whitening at supervised concentrations causes lasting damage to enamel or dentin.
Professional in-office whitening — what it actually involves
The in-office protocol is conceptually simple. The dentist isolates the gums with a protective barrier, applies a high-concentration peroxide gel (typically 25-40% hydrogen peroxide) to the teeth, and either lets it sit or activates it with a light depending on the specific product. Sessions usually run 30-60 minutes total. Most cases produce noticeable improvement after a single session; some require two sessions spaced a few weeks apart for the desired result.
The light activation that some products use is largely marketing rather than clinical benefit. The published evidence does not support light-activated systems producing meaningfully better results than the same gel without the light2, though they do warm the gel slightly which may speed reaction modestly. The patient experience is similar either way.
The advantages of in-office whitening: faster results, the dentist controls the protocol and can address sensitivity immediately, the gum protection is properly done, and the patient sees substantial change in a single appointment. The disadvantages: highest cost ($400-$1,000 per session in most US markets), higher rate of post-treatment sensitivity than at-home protocols, and the result may be slightly less stable over time than a slower, more gradual at-home protocol that allows the colour to fully stabilise.
Read also
The umbrella piece on the full cosmetic dentistry landscape. Where whitening fits in the broader picture of procedures, and how it compares to bonding and veneers for similar problems.
Professional at-home trays — the often-better option
At-home whitening with custom-fitted trays from the dentist's office is, for many patients, the better choice. The dentist takes impressions or digital scans, fabricates clear trays that fit precisely over each tooth, and provides whitening gel (typically 10-22% carbamide peroxide) to use at home. The patient applies a small amount of gel to each tray, wears the trays for 30-60 minutes (or overnight depending on the gel concentration), and continues nightly for two to three weeks until the desired shade is reached.
The advantages over in-office: substantially lower cost ($150-$600 for the trays and initial gel supply), more controlled gradual results, lower rate of sensitivity, and ability to do periodic touch-up sessions over the following years using the same trays with refill gel ($30-$60 per refill). The disadvantages: slower (results take two to three weeks instead of immediate), requires patient compliance for several weeks running, and depends on the trays fitting properly.
For most patients with naturally yellow teeth and reasonable patience, custom at-home trays produce results that are comparable to in-office whitening with less cost and less sensitivity. They are particularly good for patients who want to control the final shade — you can stop at whatever level looks natural and not over-whiten.
