Many whitening products will whiten your teeth. However, there are none that stand up to a whitening application carried out by a dentist. Your dentist is the most qualified to determine which whitening treatment is best for you. Application by a dentist ensures that your whitening treatment is applied for the exact amount of time necessary and that damage to the tooth does not occur.
When you go to a dentist the first thing that your dentist will do before whitening is perform a cleaning. This cleaning will remove tartar and plaque buildup prior to whitening which ensures a more efficient whitening process and a clean slate with which to work. When you go to have your teeth professionally whitened, to ensure that your gums are protected from unnecessary harm your dentist will apply a dental damn. The dental damn will be in place while the bleaching agent is placed on top of your teeth - which can take anywhere from 30-45 minutes. The time that a whitening agent will significantly affect the amount of sensitivity after the procedure.
Remember that when you go to have your teeth whitened by a professional, you are paying for their expertise and knowledge. The application that they provide will be much safer than an at home application, and you can ask them any questions you may have.