I started using the Express Whitening Pen for the very first time yesterday, and after the initial 3 applications (Day 1) I had already noticed a difference by bedtime. I've done my 2 applications for today (Day 2) and I'm very happy with the changes I'm seeing! I've experienced zero sensitivity, which is huge for me considering I've used other brands in the past that brought on immediate sensitivity and that alone discouraged me from using any whitening treatments at all. I'm glad I chose this brand in my decision to try again :)