There are many reasons that teeth can darken over time. First, as people naturally age their enamel begins to wear off, showing a yellowish dentin that lies beneath the surface of teeth enamel. Second, a person's food and drink plays a large role in the "yellowing" of their teeth – products like coffee, tea, and red wine are so-called "major culprits" in discoloring teeth. Tobacco also plays a very large role – the tar and nicotine found in cigarettes are very dark, and nicotine (when mixed with oxygen) turns into a yellow, staining substance. Finally, medications may play a role in darkening teeth. Certain drugs – such as antihistamines, antipsychotics, high blood pressure medications, and certain antibiotics can also discolor teeth.
First, there are whitening toothpastes. These toothpastes can help remove surface stains, which may help teeth appear whiter. However, these toothpastes do not change the color of the teeth because they are only capable of removing surface stains.
Second, someone seeking to whiten their teeth may also go to a dentist office and whiten their teeth in a so-called "chair-side bleaching" – which usually only requires one office visit. During this visit, the dentist will apply a protective gel to your gums and then apply bleach onto your teeth. The results are immediate and the teeth sensitivity is relatively minimal.
The third option involves "at-home bleaching" where a dentist creates a tray that fits your teeth. Then, at night, the patient fills the customized tray with the bleach gel and leaves it in overnight. While this may be more of a hassle than the other two options, it is certainly the most effective at removing stains and whitening your teeth.