I bought this product because I'm a coffee drinker and I wanted to get rid of some minor staining. I have never had sensitive teeth in my life, so I wasn't worried about the warning of possible sensitivity. At first I tried wearing the upper and lower strips at the same time, but it was difficult. They don't stay in place very well so it made me afraid to swallow, as they kept sliding around, but the whitening solution made me salivate so I had to keep a napkin nearby to dab at the drool. I then went to using the upper and lower strips separately, which was more comfortable (and no drool) but time-consuming. It meant an hour at a time of whitening, twice a day. After a few days I developed sensitivity in my teeth that I had never previously known, ever. Just breathing with my mouth open made me wince. I started using it only once every few days, instead of twice a day. In the end I stopped using it because it caused me too much discomfort. It did, however, whiten my teeth a couple shades and I didn't finish the box (probably used 2/3 of the strips). I would not recommend using this product, however, unless you are willing to undergo considerable pain in your teeth for a whiter smile. Like most whitening products it will cause temporary stripes or even slight transparency in your teeth for a week or so.