Whitening works by breaking down staining held within the tooth structure. That makes it effective for the gradual yellowing associated with age, diet and tobacco.
It does not change the colour of crowns, veneers or composite fillings. If you have restorations at the front of the mouth, whitening may make them more noticeable rather than less.
It is also less predictable for discolouration caused by trauma, certain medications during tooth development, or internal staining after root canal treatment. These often need a different approach.
Sensitivity is the most common side effect and is usually temporary. A supervised protocol with the right concentration and a desensitising agent reduces it considerably.
The honest summary: whitening is a good, conservative option for the right kind of staining, and the assessment beforehand is what determines whether you are in that group.
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