Most of what I write about whitening is about doing it safely and legally, and that's covered in Teeth Whitening in the UK: Simplified. This piece is about something different: the teeth that won't respond the way you hope, and why it's far better to find that out on a free call than three weeks into wearing trays.
Whitening works by peroxide diffusing through enamel and breaking down the pigment molecules in the dentine underneath. That mechanism is the whole story. Anything that isn't natural tooth, or any stain that isn't sitting where peroxide can reach it, behaves differently.
Fillings, bonding, crowns and veneers stay the colour they are
This is the one that catches people out most often. Peroxide has a much stronger effect on natural enamel and dentine than on resin composite, and porcelain is colour-stable to it. So if you have a white filling, some bonding, a crown or a veneer on a front tooth, whitening lightens the teeth around it and leaves the restoration where it was. The result is a tooth that suddenly looks darker than its neighbours, when it matched perfectly before.
Research on composite after bleaching broadly agrees: the natural tooth moves more than the restoration, which is why reviews of the topic note that front composite restorations sometimes need replacing after whitening. Newer single-shade composites blend better, but even where instruments measured no meaningful colour change, people judging the result by eye still preferred the unbleached teeth.
None of this is a reason not to whiten. It's a reason to sequence it. Whiten first, let the shade settle for a couple of weeks, then replace or add bonding matched to the new colour. That order is also why the cosmetic dentistry conversation and the whitening conversation are usually the same conversation.
Tetracycline staining: possible, but slow
Tetracycline antibiotics taken while teeth were forming, in childhood or before birth, can lock grey, brown or yellow bands into the dentine. Because the stain is deep and chemically bound, a standard two-week course rarely shifts it much.
What does work, for many people, is patience. The approach with the longest track record is low-concentration carbamide peroxide in custom trays worn nightly for months rather than weeks. Long-term studies of this nightguard approach on tetracycline-stained teeth found most of the improvement held years later. Treatment times of two to twelve months are reported, commonly three to four.
The honest caveat is the colour of the stain. Yellow and brown bands respond far better than grey or blue-grey ones, and stain sitting near the gum line is usually the slowest to move. If yours is grey and banded, it's worth knowing at the outset that whitening may get you an improvement rather than a transformation, and that bonding or veneers may be the better route.
White spots and fluorosis: whitening can make them worse
Chalky white patches, from early decay around old braces brackets or from fluorosis while teeth were forming, are a different problem. They're areas where the enamel is porous and scatters light. Whitening lightens the healthy enamel around them, and sometimes the spots themselves temporarily stand out more.
The treatment with the best evidence for these is resin infiltration: the porous enamel is gently etched and a very runny resin is drawn into it, which changes how the area reflects light. A 2021 systematic review and meta-analysis found it masked white spot lesions and fluorosis better than remineralising or fluoride varnish, with the caveat that the conclusion rests on a small number of good trials. Interestingly, for fluorosis it found that whitening first did not improve the result compared with infiltration alone.
Practical point: infiltrated enamel can pick up staining in the first weeks, so strong curries and turmeric are best kept to a minimum early on.
One dark tooth on its own
A single tooth that has gone grey, usually after a knock years ago or after root canal treatment, is almost never a whitening problem in the usual sense. The discolouration is coming from inside the tooth. Whitening the outside of the whole arch makes the neighbours lighter and leaves the dark one where it is, which makes it stand out more.
That tooth needs assessing first, with an X-ray, because a tooth that has darkened after trauma may need treatment in its own right. After that, the options range from whitening the tooth from the inside to a carefully shaded bonding or crown. Which is right depends on what's behind the colour, and that's a restorative decision as much as a cosmetic one.
Who gets the best results
| What to expect from whitening | |
|---|---|
| Yellowing with age, tea and coffee | The ideal case. Usually responds well and fairly quickly |
| Brown or orange surface staining | Often lifted by a professional clean before whitening even starts |
| Front fillings, bonding, crowns or veneers | The restorations won't change. Plan to replace them after whitening |
| Yellow or brown tetracycline bands | Can improve substantially with months of nightly tray wear |
| Grey or blue tetracycline bands | Slow and limited. Bonding or veneers may serve you better |
| White spots or fluorosis | Whitening alone can exaggerate them. Resin infiltration is the better first step |
| A single dark tooth | Needs assessing first. Usually treated on its own, not as part of the whole arch |
Sensitivity is normal, and manageable
Short-lived sensitivity is the most common side effect of tray whitening, and it's why the starting point matters. Exposed roots, worn enamel, cracks and leaking fillings all make it worse, which is one reason the law requires a dentist to examine you first. If you have tooth wear or receding gums, it doesn't rule whitening out, but it changes how it's planned: lower concentrations, shorter sessions, and a desensitising toothpaste for a fortnight beforehand.
Resources
Not sure whether your teeth will whiten? Book a free 15-minute phone or video consultation and send a photo beforehand. You'll get a straight answer about what dentist-supervised whitening is likely to achieve on your teeth, and if something else would do the job better, you'll hear that instead.
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