Another issue with using trays is dehydration of the teeth. To get equal results, tray usually must be worn for extended periods of time. The longer the trays are on, the more likely it is that your teeth will become dehydrated. The dehydration will cause the teeth to look whiter than they really are and after about a week, relapse or rebound will occur. This is where your teeth may lose up half of the whitening effect gained. Also, dehydration can lead to significant tooth sensitivity. So either way you look at it, dump the trays and get your teeth whitened quickly with a professional application pen and blue light.