I had tried the 44% and had a problem with that strength causing a lot of pain so I read some reviews on the 35% and it was also recommended by a YouTuber that had very sensitive teeth. She swore this was the only stuff that didn't cause her to have pain afterwards. I used it last night for the first time and left it on for 18 minutes. Definitely made a difference in the whiteness of my teeth, so five stars for that, sadly within a few hours I was in agony, and even had a hard time sleeping from the pain. My teeth feel better today but still a little sensitive. Just wanted people to be aware that this stuff is very strong. I'm going to try the 10% and work my way up. Great product for those who don't have my problem.