it seems as though swallowin hydrogen peroxide is unadvisable. a small amount such that you may accidentally swallow in your swishing routine would not be very harmful, and at the most, make you vomit. however, any large amount could do serious damage.

I found a great article online that proves this:

"But using hydrogen peroxide this way can be dangerous or even deadly, the U.S. Food and Drug Administration warned in July 2006. While the manufacturers’ directions call for diluting their products before use, there’s no established safe threshold for drinking or injecting hydrogen peroxide. Taken orally, the chemical might cause gastrointestinal irritation or ulcers, the agency said. Injecting it might cause inflammation, emboli (bubbles in blood vessels), and potentially life-threatening allergic reactions. And the 35 percent concentration can burn the skin or even combust.

The FDA sent warning letters in July to two Web sites marketing high-strength hydrogen peroxide for health purposes, saying the products were misbranded and sales made under those claims were illegal. As of press time, the sites were still operating but one had changed some statements.

The idea behind ingesting hydrogen peroxide stems from a now-discredited theory that cancer and AIDS thrive on a lack of oxygen in the body. Hydrogen peroxide (which has one more oxygen atom than water) supposedly provides extra oxygen that helps the body fight disease. But there’s no reliable evidence that such “oxygenation therapy” has any medical value.

CR’s take. Use only the 3 percent solution, and only for cleaning wounds. If you use it to rinse a sore in your mouth, avoid swallowing. "

http://www.consumerreports.org/health/healthy-living/home-medical-supplies/hydrogen-peroxide-12-06/overview/1206_hydro_ov_1.htm

I actually swallowed a small cup ful that had earrings in it, kinda gross yea. My parents called the poison control center and they just had me eat a couple oreo cookies and a glass of chocolate milk. Nothing bad happened and I didn't have to go to the ER or anything.

My dogs once got into some Antifreeze coolant and we weren't sure if they drank it or tasted it or what. The veterinarian told us to force them to drink Hydrogen peroxide. It does foam up a bit and I think he might have intended to help neutralize the coolant that might be in their stomachs. Otherwise is made the dogs gag and vomit a lot. I myself have gargled with H2O2 and accidentally swallowed. It wasn't pleasant but there was no harmful chemical reaction... It just triggered my gag reflex quite a lot.

It would make you throw up. It's often used to make dogs or cats throw up when they've ingested poison.

That would be a good chemistry question...I'm sure if it were to reach the stomach, it could have very bad results, as it would react with the HCl acid in your stomach and either change the PH resulting in negative effects or create some sort of other reaction that could blow you up.

I was never really good at chemistry though, so I couldn't say, but yeah...something bad I'm sure.