The outer layer of a tooth is called the enamel. The color of natural teeth is created by the reflection and scattering of light off the enamel, combined with the color of the dentin under it. Your genes affect the thickness and smoothness of the enamel. Thinner enamel allows more of the color of the dentin to show through. Having smoother or rougher enamel also affects the reflection of light and therefore the color.

Every day, a thin coating (pellicle) forms on the enamel and picks up stains. Tooth enamel also contains pores that can hold stains.

The most common reasons for teeth to get yellow or stained are:

• Using tobacco
• Drinking dark-colored liquids such as coffee, cola, tea and red wine
• Not taking good care of your teeth

Aging makes teeth less bright as the enamel gets thinner and the dentin becomes darker.

It is also possible to have stains inside the tooth. These are called intrinsic stains. For example, intrinsic stains can be caused by exposure to too much fluoride as a child while teeth are developing. Other causes include tetracycline antibiotics. They can stain a child’s teeth if taken by a mother during the second half of pregnancy or by a child who is 8 years old or younger. Teeth are still developing during these years. Trauma may also darken a tooth.

Other dental problems can affect the success of tooth whitening. For example, cavities need to be treated before teeth are whitened. That’s because the whitening solution can pass through decayed areas and reach the inner parts of the tooth. If your gums have receded, the exposed roots of your teeth may appear yellow or discolored. Whitening products will not make them whiter.

If you have tooth decay or receding gums, whitening may make your teeth sensitive. Whitening also does not work on ceramic or porcelain crowns or veneers.

Home whitening gel usually needs to be applied daily for two to three weeks. Over-the-counter kits also are widely available for home use. They provide trays to hold the gel, or whitening strips that stick to your teeth. Talk to your dentist if you want to use these home products. Be sure to follow directions to avoid overuse and possible damage to your teeth and mouth.