Mouthwash is sold as a way to rid your mouth of nasty bacteria, but a particular ingredient used in some brands mean it may not be the simple defence against cavities you might think it is.
Although this chemical is said to be 'antibacterial', that's only true in some cases. Recent findings suggest the solution can disturb the microbiome in your mouth, leading to an abundance of lactate-producing bacteria, which makes your saliva more acidic.
That's not exactly good for your teeth. Saliva plays an important role in keeping the pH of your mouth relatively neutral, but if that changes, it might cause issues in your gums and gnashers.
"There is a surprising lack of knowledge and literature behind the use of these products," says dietetics researcher Raul Bescos from the University of Plymouth in the UK.
"Chlorhexidine mouthwash is widely used but research has been limited to its effect on a small number of bacteria linked to particular oral diseases, and most has been carried out in vitro."
In the new study, 36 healthy participants were given a placebo mouthwash to use for one minute, twice a day, for seven days. Then, the next week, that fake mouthwash was swapped out with another real one that uses chlorhexidine.
At the end of each trial, the authors analysed the abundance and diversity of bacteria in each participants' mouth, measuring pH, saliva, neutralising acids, lactate, glucose, nitrate, and nitrite concentrations.
Using chlorhexidine mouthwash not only decreased microbial diversity and increased acidity, it also lowered the saliva's ability to buffer pH.
Saliva lactate and glucose concentrations were elevated after using this mouthwash, and it also disrupted the conversion of nitrate into nitrite, which may support our circulation.
This is important, because the authors found increased systolic blood pressure when the real mouthwash was used.
This weird effect has popped up in previous studies, and the authors think the use of CHX mouthwash may be more accentuated in people with high blood pressure levels.
"We have significantly underestimated the complexity of the oral microbiome and the importance of oral bacteria in the past," says biomedical scientist Louise Belfield at the University of Plymouth.
"Traditionally, the view has been that bacteria are bad and cause diseases. But we now know that the majority of bacteria - whether in the mouth or the gut - are essential for sustaining human health."
In short, this sort of mouthwash appears to be killing the good bacteria as well as the bad plaque. And while it's not clear if this is leading to oral disease, the authors say we need more information on mouthwashes before we can prescribe them correctly.
Today, that could be a very timely consideration.
"We urgently need more information on how it works on viruses."
The study was published in Scientific Reports.