We're sorry to hear that your teeth were left sensitive, and hope you're feeling much better, Irene. Some users may be more prone to sensitivity than others, as with any teeth whitening product containing peroxide, sensitivity or soreness may temporarily develop when using 3D White Whitestrips. This isn't harmful. The irritation is temporary and will subside, and there are steps you can take to reduce/eliminate this sensitivity. It is okay to use 3D White Whitestrips over a longer period of time by waiting a day between applications or alternating between using an upper and lower strip. Doing so will cause whitening to take longer, but will not cause a change in the overall whitening effect. If discomfort persists, we recommend you discontinue using the Whitestrips. So we can help you further, please give us a call at (800) 395-8423, 9 am-6 pm EST, Monday-Friday.