What is the best way to whiten unhealthy teeth?

Unhealthy? I am not sure what you mean by unhealthy. Are the teeth discolored? Do you have many broken down and or stained fillings? The first step is to go to a dentist who can address your problems and come up with the appropriate plan to restore and lighten your teeth .
Whitening. Whitening unhealthy teeth can cause a lot of sensitivity or pain. So first see a dentist and get the unhealthy teeth fixed and then whiten your teeth. Many times just by having your teeth cleaned and needed fillings done will make your teeth look much better and whiter.
See your . Dentist. Don't use OTC stuff it can hurt your teeth.