They simply don't spend enough time on your teeth to make much of a difference in the shade. That's where whitening trays and strips come in. Strips seem to come out on top in online reviews of whitening products. They adhere best to teeth without creating too much of a distasteful mess. The longer you're able to keep a whitening product on your teeth, the better the results will be. Crest 3D Whitestrips Professional Effects seem to come out on top. They're pricey at about $45 per kit, but coupons are available, and it's still a fraction of the cost of a dentist visit. Reviewers recommend cutting the strips in half lengthwise to help stretch their use. Many people experience sensitivity to hot and cold while using these products - using once a day instead of twice, as directions indicate, may help.