The peroxide sold in those brown plastic bottles is not fit for consumption. It contains “stabilizers” to give it a longer shelf life.
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That’s good to know. Where to get the kind to gargle with?
WARNING: It is often 35% H2O2, so you must dilute with distilled water before using!
I have never purchased food grade H2O2, but it appears that SunFood is the most common brand. E.g., SunFood 8 oz H2O2 for $7.99. They also have a 32 oz. size for $14.99.
My local grocery has the 32 oz. size for $9.99 (special order only). It's still a lot more expensive than the regular stuff, but hopefully more palatable.