Do your teeth hurt when you drink water that's a little cold? Or when you eat ice cream?
If not, you're fine, listen to the others in this thread.
If so...get ready baby, because the third 20-minute session (of 3 total) will feel like someone has pliers on each of your pearly whites and is literally ripping them out. Most incredible pain I've ever had, and I've been in multiple brawls. I literally saw white flashes and nearly passed out in the chair lol...it was awesome if you like that sort of thing.
The results are great though, shame because I'd love to do it again...lasts about 12 months depending upon your coffee/coke drinking habits, and whether you're a smoker.
I'll need to use trays overnight next time...sigh.