It is recommended that you continue to apply gel to your teeth at home every day for 7 days after the treatment. This prevents possible “color regression” which means that on some people some of the color can return to the teeth sooner than desired. Some of the whitening you see is caused by the dehydration that the teeth experience during the treatment, and when they rehydrate, some of the color can return (not all of it, just some). During the treatment, the pores in your enamel open up and remain open for at least 24 hours after the treatment. Since they are open, if you continue to apply whitening gel, even if it is a weaker concentration, the oxygen molecules will continue to penetrate to the dentin layer where the deep whitening takes place, which is the whitening that lasts. The most practical way of doing this is a touch-up pen, but some people elect to use a take-home kit with mouth trays.