While annoying it is an excellent indicator of wheter or not you have gotten your mouth entirely clean (for the moment). The more foaming you have the worse a job you have done, the less and the better you've done. I find that after a really good brushing I really only need to give a quick rinse so I'm done.The only thing about hydrogen peroxide that I don't like is that it foams up like crazy in your mouth. Even after I spit it out and gargle with water a few more times to get it all out, it continues to foam up all over my gums.
The foaming is in response to the peroxide killing germs or if your gums are bleeding. Once you rince well then all the peroxide will get washed away.The only thing about hydrogen peroxide that I don't like is that it foams up like crazy in your mouth. Even after I spit it out and gargle with water a few more times to get it all out, it continues to foam up all over my gums.