Q. Hi Rob, I already have the tray from my dentist and have run out of the gel- i would liek soem advice as to whether this product seems safe and also will it be effective for the amount of time that it suggest i wear it for. 23% Carbamide Peroxide – wear time : 30 minutes 35% Carbamide Peroxide – wear time : 15 minutes http://www.naturallybright.co.uk/index.php/products/view/23-teeth-whitening-gel thanks