Why Is Milk White?: & 200 Other Curious Chemistry Questions (2013)
4. Health and Safety
What happens when you swallow hydrogen peroxide?
That’s not a good thing to do. Though the body produces hydrogen peroxide as a byproduct of cellular metabolism (breathing) and has catalase enzymes to break it down, swallowing a mouthful of peroxide will overwhelm the ability of the catalase to work. This will allow the remaining peroxide to damage cells in the same way it damages bacteria. The result is a sore throat and an upset stomach.
The peroxide will also continue to break down as the body produces more catalase enzyme, so you will get bubbles of oxygen gas forming in your stomach, which will cause you to burp.