Carbamide Peroxide takes about 15 – 20 minutes to dissolve and break down in water, which then results in a smaller amount of useable Hydrogen Peroxide. It is important to note that Carbamide Peroxide is not useful for whitening teeth, only Hydrogen Peroxide is. That is why it must dissolve into Hydrogen Peroxide first.
Hydrogen Peroxide solutions are usually higher-dose, and stronger bleaching agents, and must be used with more care and due diligence.
In many home whitening kits, Carbamide Peroxide is the main ingredient because it provides for a reduced strength of Hydrogen Peroxide once broken down – this means that it is safer to use and lowers the risk of chemical burns.
If you visit a dentist, it is more likely that they will use Hydrogen Peroxide, and it is already in its ‘useful’ form without having to wait. They are trained professionals who should know the strength and safety limits of Hydrogen Peroxide.