You can use directly the strongest, as long as you use common sense involved. In practice, you start off with small amounts and short whitening time and give teeth to get used to little by little. Whitening is easy. But in practice you put a gel in your mouth tray and then allow it to be in your mouth. For detailed instructions, see the website. Generally, our customers achieve their desired whitening lever after 10 hours use. This of course varies depending on the customer used the gel strength, the desired outcome of whiteness and how yellow your teeth are initially. With more effective Gel may outcome whiter faster and the product do not have to be in the mouth for so long.