in europe it is illegal strictly speaking according to trading standards to have anything with more than 0.5% hydrogen peroxide to whiten teeth effectively you need higher concentration, they re supposedly clarifying the law but that has been case for at least 3-5 years, probably will make it illegal to get tooth whitening in beauty salons etc, the best method is at home with trays that fit over your teeth, ebest if trays are made for you by dentist with an impression tray fits better no fluid on gums etc,
the official figures and research shows at least 10% suffer sensitivity, when doing process must clean teeth first otherwise agent will not touch all of tooth surface, and while treating no heavy staining food like black coffee red wine curries with lots of tumeric etc, also will take much longer if you smoke.
officially crest stripes illegal in UK,
good whitening kit the effect should last 2-3 years less if you smoke
whitening toothpastes remove stains but not strong enough to probably whiten
whitening does not damage enamel but if sensitive can use every 2-3 days instead will just take longer, buy some sensodyne too and after brushing do not rinse just spit out