I was just going to ask that question. What concentration is advised?
I was wondering about the enamel too.
You should be less worried about the concentration of the bottle of H2O2 than the concentration in most of the whitening products you can buy, IMO
Most household hydrogen peroxide you can get in the store is going to be 3% (in the US, UK people are not able to get it I think?). So you're not going to be risking any more damage to your teeth or gums than you would be by using these products. Yes, your teeth will be sensitive after you have used it for a few days, but that is a common side effect of any hydrogen-peroxide based teeth whitening product methinks.