There are many reasons why .the teeth can become discoloured. This can include drinking tea, coffee, red wine or other food and drink with strong colourings. Another major cause is smoking, which can make the teeth appear yellowed. Tooth decay, fillings and tartar build-up can also contribute to discolouration.

Some types of staining can be caused by diseases or medicines. For example, if a type of antibiotic called tetracycline is given to children whose teeth are still developing, their adult teeth may form with a yellow or greenish tinge. There are many simple ways to improve the appearance of discoloured teeth.

There are a number of different products available to bleach the teeth. Some one can buy and use himself, while others require a visit to the dentist.

Types of teeth whitening products
There are several different ways to bleach the teeth. These are separated into external techniques (where the bleach is placed on the outside of the tooth) and internal techniques (where the bleach is placed inside the tooth).

External bleaching
This means that the colour of the teeth is lightened by placing a bleaching gel on the outer (external) surfaces of the teeth. Once again, using bleaching gels in this way may not work for certain types of discolouration, such as that caused by metal fillings or damaged blood vessels inside a tooth. There are three ways to do external bleaching.

1."Home" bleaching - this is similar to the over-the-counter kits, but with two main differences. Firstly, it is made the rubber mouth trays so that they fit the teeth precisely, and secondly, the bleaching gel is stronger so it tends to be more effective. I is given tubes of bleaching gel and instructions on how to put the gel in the mouth trays. You will need to wear the mouth trays for at least a few hours each day. It may take a few weeks to achieve the colour that you want.

2. In the dental surgery -"power" or "laser" bleaching. I is put a rubber seal around the teeth to protect the gums depending on concentration of bleaching agent used. Then, the bleaching gel is placed onto the teeth and a special, bright light is used. This light enhances the whitening process.

3. Combined bleaching - this may also be known as "power" bleaching - both the home and surgery bleaching are used in combination to obtain the desired result. The home bleaching stage may occur before or after surgery treatment - this depends on the type of product used.

Internal bleaching
This means that the colour of a tooth is lightened by placing a bleaching product inside the tooth. Internal bleaching can only be done on teeth that have been successfully root treated. This means that the blood vessels and nerves inside the tooth have been replaced with a rubber filling.

How long will the whitening last?
The whitening effects of different bleaching methods can last for a few years, but this varies from person to person. Research on dentist prescribed "home" bleaching treatment has found that for people who achieved a lighter shade, only half of them still had whiter teeth six months later. Eating, drinking, smoking and brushing habits may have an effect on how