Whitening: what works and what does not

Whitening is the treatment we are asked about most and the one with the most nonsense written about it. Here is what the evidence supports.
What works
Peroxide gel in a tray made to fit your teeth, worn for a couple of hours a day for two to three weeks. It is the same chemistry as in-surgery whitening, just slower and gentler. Results last one to three years and can be topped up.
What does not
Charcoal toothpaste scrubs stains off but also scrubs enamel, and it does not change the colour of the tooth itself. Whitening strips from the chemist use a low dose that works a little on some people and not at all on others. Blue lights mostly add drama.
Why we check first
Whitening does nothing to crowns, veneers or fillings, so if you have a white filling on a front tooth it will end up looking darker than its neighbours. It can also sting on teeth that are already sensitive. A five-minute check before you spend anything saves most of the disappointment.