For the record, I do not recall advising anyone to use hydrogen peroxide for oral treatment. Listerine, yes. H2O2, NO! If I did ever suggest this, I retract it.
A general FYI, for any chemical substance, you can search online for its "MSDS," or "material safety data sheet." These documents contain information about the hazards of a given substance, and are useful in accessing the safety of some suggested remedies.