We're sorry you experienced sensitivity. Whitestrips are very safe when used as directed, but like any whitening product that contains peroxide, temporary sensitivity may develop. We'd like to share some tips that may help in the future for preventing sensitivity. First, while using Whitestrips don't brush more than twice a day, and brush gently using a soft bristled brush. Using a mild toothpaste such as Crest Cavity Protection can also help. Another tip is to use the strips over a longer period of time by waiting a day between applications, or alternating between using an upper and lower strip. While whitening may take longer, but you'll still have the same end results. If you still experience significant irritation, then we recommend you stop use. We'd like to talk with you more about your experience, to learn how we can best help. Please give our Crest Team a call at 1-800-395-8423 from 9AM -- 6PM EST Monday–Friday. We're always glad to help!