Go to your dentist and have them professionally whitened.
The sensitivity people are talking about is from the etching of the whitening products. The enamel is etched in order to remove stains, and if you have ANY sort of decay or other issues, whitening is a bad idea. Also, if you drink lots of coke, smoke, or eat things with a high scid content (juices, tart or sour candy, fruit, etc), you don't want to risk weakening your enamel because it will make it easier for you to get further staining and cavities.
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