Some tooth discolorations may be the result of a disease or condition that requires endodontic therapy, restorations or dental surgery. Such diagnoses can only be made by a dentist or another licensed health care professional, depending on local licensing regulations. In light of these and additional factors noted below, a dental examination with appropriate radiographs or other screening or diagnostic tests is recommended prior to considering tooth bleaching. - American Dental Association report on tooth whitening
***The point here is that if you are suffering from any pain or have a sudden discoloration, this is likely a more serious problem that simple whitening will not be able to take care of. NovaWhite always recommends that you consult a dentist before whitening.