I had the Zoom whitening done last week. There is about 30 minutes of prep work before the actual procedure starts. None of the prep is painful, just awkward. Impressions are taken for custom molds to be made. Your mouth is held open and something similar to a brush on rubber is placed all over your gums so the light does not burn the gums. I had 4- 15 minute sessions, towards the end of the 3rd session the zingers started.
I'm not going to lie, they were pretty painful, my eyes watered and I dug my nails into the chair. It's not a constant pain, but a zing. I decided to go ahead with the 4th session. I was so happy when it was over. They give you a "relief" gel to put into your trays immediately after. The zingers were gone by the end of the next day. The difference in my teeth is incredible. The part that was more painful than then zingers was the no coffee, red wine, or food with dyes for 2 days!!