This works really well, much better than the 22% I previously tried due to having sensitive teeth. After using the 35% five times, I have had very little sensitivity. Make sure to start with a very small line in the trays until you figure out how much to use. If it bubbles over and burns, you have overfilled the trays. I actually use a whitening gel pen with the paint brush end that I picked up at a $1 store, and use the bristles to paint this 35% gel on my teeth and inside the trays [my trays were made by a dentist, so they have the mesh-like cushy stuff inside to hold the gel better]. That seems to help me cover the entire tooth without getting any gel on my gums.