All I can say is one time I had started using a peroxide mouth rinse (for tooth whitening), and shortly thereafter, I developed a nasty, nickel-sized sore in my mouth that stuck around for 6 weeks. It was so bad I had to see two doctors and was prescribed a strong painkiller. I didn't associate the sore with the peroxide until months later when I resumed using the mouth rinse, and developed more sores of the same type. I've since stopped using the rinse altogether and haven't had a sore since. Who knows; maybe the sores were a prelude to cancer. I'm not saying they were, but it's concerning. And even if it just causes sores, that's bad enough for me.