General whitening rarely fixes a single dark tooth, because the cause is usually inside the tooth rather than on it. That distinction changes the treatment entirely.
Often after an old knock, sometimes years earlier. The tooth darkens from within.
Internal bleeding after trauma
Discolouration from blood products within the dentine.
A large old restoration
Metal or dark composite showing through thin enamel.
Decay under a restoration
Shadowing through the tooth.
Staining on one surface only
The simplest case, and the most easily treated.
What the options usually are
If the nerve is dead, external whitening will not lighten it meaningfully; internal whitening after root canal treatment, or a restoration such as a veneer or crown, are the usual routes. If a restoration is the cause, replacing it may be enough.
The first step is a test and an X-ray, because the answer depends entirely on which of these applies.
What we can explain here — and what needs an exam
A page can inform you. It cannot examine you.
We can explain online
Why a single tooth darkens
Why external whitening often does not help
The general routes available
Determined after an exam
Whether the nerve is alive
Whether internal whitening is viable
Whether a veneer or crown is more appropriate
What to do next
A vitality test and X-ray answer this quickly. Do not spend months on whitening strips first.