Natural tooth color is a genetic trait that varies from person to person, similar to hair and skin color. Some teeth are more yellow, and some yellow with aging. There are two different types of tooth discoloration; surface discoloration, and internal discoloration. Although, bleaching successfully lightens most discoloration, certain types (mostly internal) are more difficult to remove. The following are causes of tooth discoloration (internal and external):

External: Tobacco Beverages such as coffee and tea Certain foods, such as berries Deposits of calculus or tartar

Internal: Aging Tramatic injury Excessive fluoride Certain illnesses Taking the antibiotic tetracycline during early childhood

There are two types of bleaching procedures. Bleaching may be done completely in a dental office (known as chair side bleaching), or a system may be dispensed by the dentist for the patient to use at home (home-use bleaching) 

Chair side bleaching is done in the dental office and usually takes anywhere from 30 minutes to 1 hour per visit (as with any procedure, consult your dentist for exact times). It is not uncommon for the teeth to become slightly sensitive following treatments.

To protect the mouth, a gel-like substance may be applied to the gums and a rubber "shield" is placed around the necks of the teeth. A chemical solution, the oxidizing agent, is then "painted" on the teeth. A special light may be used at five-minute intervals to help activate the agent.

Home-use Bleaching is done by the patient under the supervision of the dentist while at home. In home-use bleaching, you are given a mouth guard that has been made from an impression of your teeth. You are then given a gel that is inserted into the mouth guard, and is worn (mouth guard with gel) for up to two hours daily or at night for about two weeks. The duration may vary according to your needs and your dentist's recommendations.

Some people with certain dental conditions may not be good candidates for this type of bleaching. People with gum recession, for example, may have exposed root surfaces that are highly sensitive that could be further irritated by the ingredients in whiteners. Also, these products are not recommended for habitual tobacco and alcohol users.

Please consult your dentist for any other procedures and conditions that may be recommended for your bleaching.

Over-the-counter products that are self-administrating are not recommended. Bleaching treatments should only be done under the supervision of a dentist following a proper examination and diagnosis of the discolored teeth.

In addition, if you have had tooth-colored crowns, bonding or composite restorations placed, talk with your dentist, since these materials will be a different shade of color than newly bleached teeth.

Tooth bleaching doesn't require any tooth structure to be removed, as do some other procedures to whiten teeth, and it is also the most economical. 

Teeth may be bleached a shade lighter than desired since they will tend to darken slightly with time.
Most bleachings last one to three years, although in some situations it may be longer.

Your dentist will monitor the entire process of home-use bleaching to ensure its effectiveness and safety. Whiteners with the ADA Seal of Acceptance have been proven to be safe and effective. Consult your dentist about the whiteners that they use.