In-office teeth whitening comprises a series of one-hour appointments at a dentist's office. Teeth will be examined and cleaned to make sure there are no cavities, loose fillings, gum problems or other issues that may be exacerbated by the bleaching process. Whitening procedures are effective but their results are not permanent. Depending on the procedure used, whitening may fade over time. Patients are advised to avoid problem food and drinks that may stain or darken the teeth.