@chian It’s the peroxide. Did they hurt the first time? If not you could do it two days in a row, and then stop for a week, and then two days again, for three or four weeks, or however long is good for you. Make sure you don’t use peroxide (whitening) toothpaste or mouthwash while doing it, and really ever. Toothpaste and mouthwash are only on your teeth for a minute and barely have time to really whiten anything, but if you have sensitive teeth, it wil keep your teeth in a state of being more sensitive. Not sure if this will help, but thought I would mention it.