Can mouthwash containing hydrogen peroxide be harmful to your mouth?

At normal use, the risks of hydrogen peroxide mouthwash are small. A study with 45 people who rinsed twice a day for 90 days with a concentration of 2.5% found only very mild tooth sensitivity and no gum irritation. That is a reassuring finding for daily use at that concentration.

Even so, some questions remain unanswered. A 2020 review pointed out that possible effects on the mouth's bacterial balance, irritation of the mucous membranes, and disturbances in smell were barely measured in ongoing research at the time. That does not mean those risks definitely occur, but they have not been properly mapped out either. So it is not yet clear what can happen with long-term use or higher concentrations.

As for tooth staining: this is mentioned as a possible risk in the review, but how often it occurs at everyday concentrations does not emerge from the studies available here.

On other points, hydrogen peroxide in the mouth performs better or worse than expected. As a pre-rinse before a dental procedure, chlorhexidine tackles bacteria in aerosols noticeably more effectively than hydrogen peroxide. And a claim you sometimes come across, that rinsing with hydrogen peroxide kills viruses in your saliva, has not been demonstrated so far.

In short: at low concentrations (around 2.5%) and normal use, hydrogen peroxide mouthwash appears to be safe and to have few side effects. Higher concentrations and longer-term effects have been too little studied to say anything definitive about them. If you experience tooth sensitivity or sensitive gums, it is wise to discuss its use with your dentist.