From an ABC News article on teeth whitening:
O’Hara’s dentist, Dr. Joseph Kravitz, says many patients don’t realize that overexposing their gums to peroxide can cause painful complications.
“When you leave it on too long, it causes inflammation of the gum tissue. The chemicals cause the gums to recede, exposing the roots of the teeth. If you’re using these treatments and your gums to turn red or purple or you have pain, you should stop,” he says.
Here’s an additional video interview of Dr. Kravitz on teeth whitening.
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