There are many over the counter whiting strips that are for the most part are affordable. From what I can tell they are all about the same so it is about choosing an name brand that you feel good with. With that being said this is what I also been able to find out about whitening strips
Some research suggest that whitening strips may have side effect such as gum irritation and tooth sensitivity, especially for first time users; with that in mind one may want to start off with shorter time used or a less concentration of whitener. If you are still experiencing the side effect you may wish to stop the use of the strips.