Antiseptic mouthwash reduces bacteria. The surprising part is what happens next.
The logic behind antibacterial mouthwash seems almost self-evident.
Dental plaque contains bacteria. Bacteria contribute to oral disease. Antiseptics kill bacteria. Therefore, killing more bacteria should make the mouth healthier.
Chlorhexidine (CHX), one of the most effective oral antiseptics, provides a useful test of that assumption. It certainly suppresses bacteria and plaque. It also reduces gingivitis [7].
But the rest of the story is considerably stranger.
Human experiments show that CHX can disrupt microbial functions that appear beneficial to us. It strongly suppresses bacterial nitrate metabolism [1,2]. It changes salivary acidity, lactate and buffering [3]. It can substantially alter the composition and diversity of the oral microbiome [3,11].
Yet reducing bacteria does not reliably translate into less tooth demineralization or less caries [4–6].
We therefore started with one question:
Does killing oral bacteria actually make the mouth healthier?
The evidence led us to a different one:
What useful functions disappear when we disturb the microbial ecosystem?
1. Our original hypothesis was too simple
Some oral bacteria reduce nitrate to nitrite, the first step in the enterosalivary nitrate–nitrite–nitric oxide pathway.
If a broad-spectrum antiseptic eliminates these bacteria, nitrite availability might fall, nitric oxide signalling might be affected, and blood pressure might rise.
There is unusually direct experimental evidence for the beginning of this chain.
In 19 healthy volunteers, seven days of antiseptic mouthwash reduced oral nitrite production by about 90% and plasma nitrite by 25%. Systolic and diastolic blood pressure increased by approximately 2–3.5 mmHg. Changes in circulating nitrite were associated with the blood-pressure response [1].
This suggested a simple mechanism:
CHX → fewer nitrate-reducing bacteria → less nitrite → less NO → higher blood pressure
But later microbiome experiments exposed a problem with that explanation.
2. More “useful” bacteria, less useful function
A randomized study followed 45 healthy participants. Twenty-one used CHX and 24 used propolis mouthwash twice daily for seven days.
Researchers measured both the relative abundance of species classified as nitrite producers and actual nitrite-producing activity.
After CHX, the relative abundance of presumed nitrite-producing species increased.
At the same time, actual nitrite-producing activity decreased substantially.
Both effects were statistically strong (p < 0.001) [2].
That gives us an apparent paradox:
more presumed nitrite-producing bacteria → less nitrite production
Sequencing can tell us which organisms are present. It cannot automatically tell us what the microbial ecosystem is doing.
A bacterium capable of performing a biochemical reaction does not necessarily perform it at the same rate after its ecological environment has changed.
Our original hypothesis was therefore too crude.
CHX can disrupt microbial function even when organisms associated with that function remain present.
3. The disruption is not limited to nitrate
A crossover experiment in 36 healthy participants examined the oral microbiome and several biochemical markers after seven days of CHX.
Microbial diversity decreased. Salivary pH and buffering capacity decreased. Lactate and glucose increased. Salivary and plasma nitrite decreased [3].
The increase in systolic blood pressure in this experiment was not statistically significant [3].
The oral microbiome is therefore not merely a list of organisms living in the mouth. It is also a chemical system.
Change the ecosystem and several of its outputs can change simultaneously.
4. Did the altered chemistry damage teeth?
This seemed like an obvious next hypothesis.
If CHX produces a more acidic environment, reduces buffering and increases lactate, perhaps disturbing the microbial ecosystem paradoxically increases demineralization.
We looked for direct evidence. The hypothesis did not survive.
In a randomized in-situ experiment, 14 participants carried enamel and dentine specimens in their mouths for four weeks and rinsed twice daily with either 0.2% CHX or saline.
CHX did not provide significantly better protection against enamel or dentine demineralization than saline. But it did not significantly worsen demineralization either. Plaque from the two conditions also showed no significant difference in organic-acid production after a sucrose challenge [4].
The result is therefore not “CHX damages teeth.”
It is: killing more bacteria did not automatically protect teeth.
5. What about actual caries?
Longer clinical trials provide an even stronger test.
A double-blind placebo-controlled trial recruited 1,101 adults aged 60–75 and followed tooth surfaces annually for up to five years.
For sound coronal surfaces, the hazard ratio for becoming filled, decayed or extracted with CHX was 0.87 (95% CI 0.71–1.14; p=0.20). For root surfaces it was 0.91 (95% CI 0.73–1.14; p=0.41). Neither difference was statistically significant [5].
A systematic review reached a compatible conclusion. CHX mouthwash changes Streptococcus mutans levels, but evidence that this translates into less actual caries remains insufficient [6].
antimicrobial effect ≠ anticaries effect
6. But CHX is not useless
Here the strongest version of our initial suspicion fails.
A Cochrane review covering 51 studies and 5,345 participants found high-quality evidence that CHX used alongside mechanical oral hygiene produces a large reduction in dental plaque. It also reduces gingivitis in people with mild gingival inflammation, although the absolute gingivitis reduction in that group was not considered clinically important [7].
CHX therefore has genuine clinical effects. It is not merely cosmetic.
This leaves us with a trade-off rather than a villain.
CHX can suppress plaque-associated processes while simultaneously disturbing other microbial functions whose importance is less certain.
Short-term CHX use for a specific dental indication is therefore not biologically equivalent to indefinitely suppressing the oral microbiome of a healthy person.
7. Are oral bacteria actually protecting our teeth?
Some microbial functions may contribute to protection.
Oral bacteria can metabolize arginine and urea to generate ammonia, helping neutralize acids and regulate plaque pH [8–10].
Human observational studies have found substantially greater alkali-generating activity in caries-free people than in caries-active groups [8,9].
These findings do not prove that increasing these functions will prevent caries in every person. But they undermine the simpler model in which bacterial abundance itself is the enemy.
The better ecological model is neither “bacteria cause caries” nor “bacteria prevent caries.”
Different microbial functions push the ecosystem in different directions.
Frequent carbohydrate exposure can favour acid-producing and acid-tolerant communities. Other microbial pathways generate alkali and help restore pH [8–10].
The relevant variable may therefore not be bacterial quantity. It may be ecological function.
MOUTHWASH
COMMUNITY STRUCTURE
plaque ↓
community composition changes
nitrate → nitrite ↓
pH / buffering altered
gingivitis ↓
long-term systemic effects ?
Strong evidence for the earlier links; evidence becomes more limited and mixed toward long-term clinical outcomes.
MICROBIAL ABUNDANCE ≠ MICROBIAL FUNCTION ≠ HOST HEALTH
8. The microbiome does not necessarily reset when CHX stops
A longitudinal shotgun-metagenomics study followed 20 periodontal patients using 0.2% CHX twice daily for four weeks after periodontal procedures.
Saliva and supragingival plaque were sampled before CHX, after four weeks of treatment, and four weeks after treatment stopped.
Alpha diversity fell significantly during CHX exposure, and community composition changed in both saliva and plaque. Streptococci became particularly prominent during treatment [11].
After CHX stopped, several conspicuous changes moved back toward baseline. Recovery, however, was not identical between saliva and plaque, with plaque showing evidence of less complete ecological recovery [11].
This supports incomplete short-term recovery, not permanent damage.
The study followed participants for only four weeks after cessation. Its participants also had periodontal disease and underwent periodontal procedures. It therefore cannot tell us whether a similar disturbance persists long term in healthy people.
Persistence is evidence of perturbation. It is not automatically evidence of harm.
9. Blood pressure turned out to be the weaker story
Our investigation began with blood pressure. It ended up being one of the less convincing clinical outcomes.
The nitrate mechanism itself is experimentally well supported [1–3]. Individual experiments have reported small blood-pressure increases after antiseptic mouthwash [1].
But a 2026 systematic review and meta-analysis identified only five eligible studies and found no statistically significant overall effect.
For systolic blood pressure: +1.56 mmHg (95% CI −0.67 to +3.80; p=0.17).
For diastolic blood pressure: −0.10 mmHg (95% CI −1.99 to +1.78; p=0.91) [12].
The authors concluded, with moderate confidence, that any blood-pressure change attributable to CHX in patients for whom it is recommended is insignificant or very small [12].
A measurable disruption of a biological pathway does not guarantee a clinically important downstream effect.
10. What survived our attempts to falsify the hypothesis?
We began with:
Broad-spectrum antiseptic mouthwash may harm health by killing beneficial oral bacteria.
That formulation did not survive intact.
The evidence supports something more subtle:
Broad-spectrum antiseptic mouthwash can disrupt potentially beneficial functions of the oral microbial ecosystem, even when organisms associated with those functions remain present. Whether these disturbances cause important long-term harm is not established.
Several findings appear reasonably robust. CHX strongly suppresses plaque [7]. It reduces gingivitis under some conditions [7]. It changes oral microbial composition and diversity [3,11]. It can strongly suppress nitrate-to-nitrite metabolism [1–3]. It can alter salivary pH, buffering and lactate [2,3]. Simply suppressing bacteria does not reliably prevent tooth demineralization [4]. Long-term clinical evidence does not demonstrate a substantial anticaries effect from CHX mouthrinse [5,6]. And some ecological changes remain detectable after treatment stops [11].
But several stronger claims did not survive.
We found no convincing evidence that CHX mouthwash increases caries. We found no convincing evidence that it produces a clinically meaningful increase in blood pressure [12]. We found no evidence that its microbiome effects are permanent. And we found no adequate causal evidence that these microbiome changes produce long-term systemic disease.
11. The more general result
The mouthwash question exposes a broader problem in microbiome research.
It is tempting to classify organisms as good or bad and then count them.
The evidence here shows why that can mislead.
A microbial community can contain more organisms thought capable of a useful reaction while performing less of that reaction [2]. Conversely, reducing bacterial abundance does not guarantee improvement in a downstream health outcome [4–6].
This leaves three distinct quantities:
microbial abundance ≠ microbial function ≠ host health
They interact. They are not interchangeable.
For an antiseptic, therefore, one of the least informative questions may be: How many bacteria does it kill?
A better one is: What does the ecosystem stop doing after we kill them?
And the question the current evidence still cannot answer is the most important one:
Does that functional disruption matter to human health in the long run?
For now, that is where the evidence ends.
References
- Kapil V, Haydar SMA, Pearl V, et al. Physiological role for nitrate-reducing oral bacteria in blood pressure control. Free Radical Biology and Medicine. 2013;55:93–100. doi:10.1016/j.freeradbiomed.2012.11.013.
- Bescos R, du Toit L, Redondo-Rio A, et al. The comparative effect of propolis and chlorhexidine mouthwash on oral nitrite-producing bacteria and blood pressure regulation. Journal of Oral Microbiology. 2025;17(1):2439636. doi:10.1080/20002297.2024.2439636.
- Bescos R, Ashworth A, Clarke C, et al. Effects of Chlorhexidine mouthwash on the oral microbiome. Scientific Reports. 2020;10:5254. doi:10.1038/s41598-020-61912-4.
- van Strijp AJP, Gerardu VAM, Buijs MJ, van Loveren C, ten Cate JM. Chlorhexidine efficacy in preventing lesion formation in enamel and dentine: an in situ study. Caries Research. 2008;42(6):460–465. doi:10.1159/000170587.
- Wyatt CCL, Maupomé G, Hujoel PP, et al. Chlorhexidine and preservation of sound tooth structure in older adults: a placebo-controlled trial. Caries Research. 2007;41(2):93–101. doi:10.1159/000098041.
- Coelho ASEC, Paula ABP, Carrilho TMP, et al. Chlorhexidine mouthwash as an anticaries agent: a systematic review. Quintessence International. 2017;48(7):585–591. PMID: 28555200.
- James P, Worthington HV, Parnell C, et al. Chlorhexidine mouthrinse as an adjunctive treatment for gingival health. Cochrane Database of Systematic Reviews. 2017;(3):CD008676. doi:10.1002/14651858.CD008676.pub2.
- Nascimento MM, Gordan VV, Garvan CW, Browngardt CM, Burne RA. Correlations of oral bacterial arginine and urea catabolism with caries experience. Oral Microbiology and Immunology. 2009;24(2):89–95. doi:10.1111/j.1399-302X.2008.00477.x.
- Gordan VV, Garvan CW, Ottenga ME, et al. Could alkali production be considered an approach for caries control? Caries Research. 2010;44(6):547–554. doi:10.1159/000321139.
- Liu Y-L, Nascimento M, Burne RA. Progress toward understanding the contribution of alkali generation in dental biofilms to inhibition of dental caries. International Journal of Oral Science. 2012;4(3):135–140. doi:10.1038/ijos.2012.54.
- Bartsch S, Kohnert E, Kreutz C, et al. Chlorhexidine digluconate mouthwash alters the oral microbial composition and affects the prevalence of antimicrobial resistance genes. Frontiers in Microbiology. 2024;15:1429692. doi:10.3389/fmicb.2024.1429692.
- Toonen LSJ, Van Swaaij BWM, Vagevuur ML, et al. The Effect of Chlorhexidine Mouthwash on Blood Pressure: A Systematic Review and Meta-Analysis. International Journal of Dental Hygiene. 2026;24(3):390–403. doi:10.1111/idh.70035.
Article type: Research Article · Evidence Review
Scope note: This is a structured evidence review, not a systematic review. The investigation followed a falsification-oriented sequence of questions and prioritised human intervention studies, direct clinical outcomes, systematic reviews and longitudinal microbiome data. It should not be interpreted as an exhaustive search of all mouthwash formulations or all possible health outcomes.