There's not a lot of research on the benefits and risks of gargling with hydrogen peroxide, but it's possible to do more harm than good. We need "good" bacterial in our mouths for healthy teeth, gums and the overall microbiome. Mouthwashes that contain alcohol kill the good microbes along with the bad, so experts (and research) question it's use, and one can logically extend that caution to hydrogen peroxide. If you choose to do it, though, be sure to dilute it considerably, and don't swallow it. I would recommend searching around to see what some dentists and respected health sites say — you might be less inclined to continue.