DIY bleaching

You can buy products to bleach your teeth whiter over the counter at a pharmacy or online. They use hydrogen peroxide (or carbamide peroxide, which breaks down to hydrogen peroxide) to bleach stains and the tooth itself.

• Paint-on gels: These are applied once or twice a day for up to two weeks. The taste is not unpleasant - but the gel may rub off if it hasn't dried before you close your mouth.
  
• Tray-based whiteners (pictured right): Gel is placed in a tray (mouthpiece) and worn for up to 15 minutes twice a day for two weeks. Over-the-counter trays are unlikely to fit properly and may irritate the gum if the whitening agent seeps out.
• Strips: These are worn on your teeth for 30 minutes twice daily for two weeks. They're more convenient than trays, but they don't provide full coverage (they leave your back teeth and maybe the areas next to your gums unbleached).
   

An overview of bleaching product user trials by Australia's Cochrane Collaboration found that these products work better than a placebo or no treatment. The trials covered tray, strips and paint-on-gels. 

Safety

Bleaching may cause gum irritation and sensitivity to heat or cold, although these side effects go away soon after you stop the treatment.

Bleach can also penetrate cracks or cavities in your teeth - and high concentrations can cause irreversible damage to the tooth pulp and enamel, as well as irritating the gums and lining of the mouth and weakening filling materials. 

• Make sure you have your teeth checked by a dentist before starting a DIY bleaching treatment. If you have cavities or failing dental work you're more likely to experience irritation and sensitivity.
• Don't be tempted to keep going with a DIY bleaching treatment for longer than 14 days. There's a point beyond which your teeth won't get any whiter - and continuous bleaching can damage tooth enamel. Wait at least three months between treatments.

The bottom line

These products are more effective at removing surface stains than "whitening" toothpastes. For any significant staining, it's better to see a dentist.