I have used Crest Whitestrips several times, all with great results. I prefer the 14 day strips because the 7-day or 3-day packs tend to make my teeth and gums extremely sensitive. I experience no side effects with the 14-day strips, only significantly whiter teeth. The goo in the strips is somewhat unsavory, but well worth the results. This is the only brand of whitening strips I will stand… read full review »