Some dentists recommend using hydrogen peroxide as a mouth rinse, but others say to stay away from it all costs because of potential health hazards, including mutagenity. In searching through a number of toxicology, chemical, and medical texts, its difficult to determine the reason that a dentist was so adamant that you not use hydrogen peroxide because of its mutagenic potential.
Hydrogen peroxide is represented by the chemical formula H2O2. The pure chemical product is often used in industry, but 100% hydrogen peroxide is not available to the general public. Most often hydrogen peroxide that can be purchased at a drugstore has a concentration of 3% in water (that is, 3% H2O2, 97% H2O).
The NJ Factsheet on hydrogen peroxide clearly states that a 100% concentration, it is a mutagen (able to cause damage within the structure of the cell, at the DNA level). However, the factsheet further states that the listed health effects are unlikely to occur with commercial solutions (by which they mean the concentration available at the drugstore, which is usually a 3 to 5% solution in water). In other words, they are saying that people who must work with hydrogen peroxide at a 100% concentration must avoid contact because at that concentration it is clearly hazardous, but is unlikely to cause harm at low concentrations.
In addition to the information provided by the NJ Factsheet, the Merck Index (page 633) provides information on its chemical properties. Hydrogen peroxide is not a very stable compound. It must be protected from light and kept in a cool place, because when it is exposed to light and heat it reacts to become water and oxygen (H2O2 --- (light + heat) ---->H2O + O2). This reaction is not hazardous to your health. Hydrogen peroxide is also affected by the action of swishing, as in a mouthwash, and by simply coming into contact with surfaces, such as your teeth. What this means is that once hydrogen peroxide goes into your mouth, the conditions are very good for it to react rapidly to degrade to water and oxygen. In fact, the bubbles that you feel in your mouth are oxygen bubbles that are moving through the water so they can escape into the air, much like the carbonation in soda pop. In other words, once hydrogen peroxide has been removed from the bottle, it doesn't remain hydrogen peroxide very long. In the event that you may accidentally swallow a small amount of the hydrogen peroxide, the heat in your body and acid in your stomach should cause it to react before it can be absorbed by your body.
Finally, the last issue to be addressed is the fact that your mouth does not come in direct contact with hydrogen peroxide for a short period of time before it degrades to water and oxygen. In the chapter on disinfectants and antiseptics in Basic & Clinical Pharmacology (Lange, 1984), it notes that hydrogen peroxide does not penetrate tissue, even when used on open wounds, cuts or sores. Tissue penetration is required for an effect to take place at a cellular level. Without tissue penetration, a mutation can't occur.