By Simply Health Daily Editorial · October 2026 · How we source and correct
Antiseptic mouthwash does what it says. That is the whole question.
A rinse formulated to kill oral bacteria does not distinguish between the species you would rather be without and the ones doing useful work. In 2020 a research group at the University of Plymouth measured what that actually looks like, and the results are worth understanding before you decide how often to reach for the bottle.
The study
Bescos and colleagues published their findings in Scientific Reports. Thirty-six healthy adults completed a crossover protocol: seven days rinsing with a placebo, then seven days rinsing with 0.2% chlorhexidine — twice a day, one minute each time. Saliva and blood were sampled throughout.
Chlorhexidine is not the mild cosmetic rinse sold next to the toothpaste. It is the prescription-strength antiseptic dentists use after surgery and for managing gum inflammation. It is a reasonable thing to study precisely because it is potent.
What changed
The community shifted. After a week of chlorhexidine, Firmicutes and Proteobacteria increased significantly. Bacteroidetes, TM7 and SR1 decreased, as did Fusobacteria. This was not a uniform knockdown — it was a reshuffling, with some groups gaining ground precisely because competitors had been cleared out.
Diversity fell. The Shannon index, a standard measure of how varied a microbial community is, dropped significantly. In ecology, falling diversity generally means a less resilient system.
The mouth got more acidic. Salivary pH fell, and so did buffering capacity — saliva’s ability to neutralise acid. Salivary lactate and glucose both rose.
That last cluster deserves a pause. Acid is what dissolves enamel. Buffering capacity is one of the defences against it. A rinse marketed for oral health moved both in the wrong direction over seven days.
The nitrate finding
This is the part that drew attention beyond dentistry.
Certain oral bacteria reduce dietary nitrate — from leafy greens and beetroot — into nitrite. You swallow that nitrite, and the body converts some of it into nitric oxide, a signalling molecule involved in relaxing blood vessels. Humans do not have an efficient enzyme for that first step. We outsource it to bacteria living on the tongue.
In the study, chlorhexidine reduced the oral nitrate-reducing capacity. Salivary nitrate went up, because less was being converted. Salivary and plasma nitrite both went down.
Here is where to be careful. Systolic blood pressure did rise after chlorhexidine — but the increase was not statistically significant. The study demonstrates an interrupted biochemical pathway. It does not demonstrate that mouthwash raises your blood pressure, and you should treat any article claiming it does as having overread the data.
What the authors themselves flag
The paper lists its own limitations plainly, which is a good sign:
- The design was not randomised, and placebo always came first.
- There was no washout period between the two phases.
- Only saliva was sampled, so site-specific communities — plaque, the gum crevice — were not assessed separately.
- Oral health was assessed by questionnaire rather than clinical examination.
- Thirty-six people is a reasonable size for this kind of work, not a large one.
It is one well-conducted study of a prescription-strength antiseptic in healthy adults over seven days. It is not the final word.
What this does not mean
It does not mean chlorhexidine is bad. It is prescribed for specific reasons — after oral surgery, during active periodontal treatment — where the short-term benefit clearly outweighs a temporary microbial shift. If a dentist has prescribed it, this study is not a reason to stop.
It does not transfer automatically to every mouthwash. The study tested 0.2% chlorhexidine. Over-the-counter rinses vary enormously — some antiseptic, some essentially flavoured water, some built around fluoride, which is doing something different entirely.
It does not mean rinsing is pointless. It means a rinse is an intervention with ecological consequences, and that “kills 99.9% of germs” describes a mechanism, not a benefit.
The reasonable reading
Daily antiseptic rinsing, indefinitely, as general maintenance in a healthy mouth is harder to justify than the marketing suggests. There is a real trade-off, it has now been measured, and it was not obvious before.
If you use an antiseptic rinse because a dentist told you to, keep using it as directed. If you use one out of habit, it is a fair question to raise at your next appointment — along with what, specifically, you are trying to achieve.
Sources
- Bescos R, Ashworth A, Cutler C, et al. Effects of Chlorhexidine mouthwash on the oral microbiome. Scientific Reports, 2020;10:5254.
- Krishnan K, Chen T, Paster BJ. A practical guide to the oral microbiome and its relation to health and disease. Oral Diseases, 2017;23(3):276–286.
This article is general information, not medical or dental advice. It is not intended to diagnose, treat, cure or prevent any disease. Talk to a dentist or doctor about your own situation. Simply Health Daily is reader-supported and earns commissions on some links, at no extra cost to you — see our Affiliate Disclosure.