I read the reviews and I was a little nervous about the pain. You can do up to 4, 15 min rounds. The first round was fine, didnt hurt at all, the second round I got the ZING that people talk about but it still wasnt bad. the 3rd round was TERRIBLE! I had to stop half way in. My teeth did come up 2/3 shades, I am excited to try the take home trays to see if they come up a little more. I personally would not go through the pain again, it was worse than child birth. I cried all the way home, the pain 96% gone by the next morning. My doctor was amazing the pain was just really bad.