35% fast action. I left it on for 30 minutes twice and my teeth got the desired whiteness. Mind you, I wasn't aiming for sparkling white false teeth. The advantage of 35% over 20% is that it doesn't need to be used for multiple hours. The downside is you have to be VERY careful not to put too much on the mouthguard lest it splatter on your gums. You will regret as there will be a burning sensation.
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