Can't vouch for the pain. Before you head to the dentist you may want to read this article from NPR.org on teeth whitening to find out if one of those bleaching systems is for you.
Here's an excerpt ...
"Chemist Lee Hansen, a professor emeritus at Brigham Young University, explains there was an assumption that heat from the light served as a catalyst to decompose the bleaching gel.
"That's the theory behind it," says Hansen. But he found the lights don't generate enough heat or give off enough UV light to accelerate the chemical reaction."
How soon are you planning on going in?