Does Mouthwash Kill Good Bacteria? What the Research Actually Shows

Last updated: October 2026 · 9 min read · By Debora Vera

Independent review. I am not a dentist or a researcher. I read labels and check claims against published research. This article contains no affiliate links.

The short answer

Yes. Antiseptic mouthwash does not only hit harmful bacteria: a 2023 review describes how chlorhexidine can kill certain species and leave others, sometimes unwanted ones, to take over. But the best-known findings come from small, one-week studies of that one ingredient. Microbiome research on other common ingredients is limited, and dentists may still recommend a rinse for good clinical reasons.

Why the bacteria in your mouth matter

Foods like beetroot and leafy greens are rich in nitrate, and that nitrate shows up in your saliva. Certain mouth bacteria convert it into nitrite. A 2005 study in the European Journal of Oral Sciences found this conversion is concentrated at the back of the tongue’s upper surface, with Veillonella species the most common nitrate reducers it isolated.

Your body can then use nitrite to make nitric oxide. A 2023 review in the International Dental Journal links this pathway to cardiovascular health, and a University of Liverpool oral biologist notes that nitrite widens blood vessels, which lowers blood pressure. Nearly every headline linking mouthwash to blood pressure traces back to this pathway.

The nitrate to nitrite to nitric oxide pathway in the mouth, with bacteria at the back of the tongue handling the conversion step

What the chlorhexidine studies found

In a 2013 study in Free Radical Biology and Medicine, researchers at Queen Mary University of London followed 19 healthy volunteers through a one-week control period and then one week of chlorhexidine mouthwash. Oral nitrite production fell by about 90%, blood nitrite by about 25%, and blood pressure rose by roughly 2 to 3.5 mmHg, starting within one day.

A 2020 study in Scientific Reports from the University of Plymouth had 36 healthy adults rinse for one minute twice a day: one week with a placebo, then one week with 0.2% chlorhexidine. The chlorhexidine week shifted the salivary microbiome (more Firmicutes and Proteobacteria, fewer Bacteroidetes, TM7, SR1 and Fusobacteria), lowered saliva pH and reduced nitrite. Systolic blood pressure rose, but the change was not statistically significant.

These are real, measured effects. They also come from small groups, over one week, using one ingredient. In 2014, a lecturer in oral biology at the University of Liverpool wrote that the 2013 paper’s core findings appeared credible, but that the extrapolation to blood pressure was perhaps less clear.

What about other mouthwash ingredients?

Both studies above tested chlorhexidine, a strong antiseptic. In the UK, chlorhexidine mouthwashes are licensed for 30 days of use and are not recommended for routine use, according to the UK’s National Institute for Health and Care Research.

Other common rinses use different active ingredients, such as essential oils or cetylpyridinium chloride (CPC). Listerine Cool Mint, for example, lists eucalyptol, menthol, methyl salicylate and thymol as active ingredients on its US label. The 2023 review found microbiome evidence on CPC limited, noting that one placebo-controlled study of six weeks of twice-daily CPC rinsing found no significant change to the oral microbiome. For essential oils, the authors found no studies of their effects on the oral microbiome in people.

A 2024 crossover trial in the Journal of Medical Microbiology, run by researchers in Antwerp, Belgium, had participants use Listerine Cool Mint daily for three months and a placebo for three months, in either order. After the Listerine period, two opportunistic bacteria, Fusobacterium nucleatum and Streptococcus anginosus, were more abundant in throat swabs than at baseline.

The details matter. The 59 participants were men who have sex with men taking HIV pre-exposure prophylaxis, enrolled in a trial about preventing sexually transmitted infections. Samples came from the throat, not the teeth or gums. The placebo period also changed the microbiome compared with baseline. And the study measured bacteria, not disease. The lead author said that, ideally, long-term use should be guided by healthcare professionals.

A 2017 study in Nitric Oxide followed 945 overweight or obese adults aged 40 to 65 in San Juan, Puerto Rico, for three years. Those who used mouthwash at least twice a day had about a 55% higher risk of developing prediabetes or diabetes than less frequent users. Use less than twice a day showed no association. Because the study was observational, it shows an association, not cause and effect.

Table comparing chlorhexidine, essential oil, CPC and fluoride rinses by how much microbiome research exists and what it showed

Where the evidence splits

There is a strong case on the other side too. A 2017 Cochrane review of 51 studies with 5,345 participants found that adding chlorhexidine rinse to regular cleaning produced a large reduction in dental plaque at 4 to 6 weeks and at 6 months. The improvement in gum inflammation for people with mild gingivitis was small, and the authors did not consider it clinically relevant. Rinsing for four weeks or longer caused tooth staining.

The American Dental Association says mouthwash is not a replacement for daily brushing and flossing, but may be helpful for some people. Dr. Hsun-Liang Chan, chair of periodontology at The Ohio State University College of Dentistry, has said mouthwash is, in general, safe to use, and that while it can change the balance of mouth bacteria, there is not much evidence this is cause for concern, especially with temporary use.

So the disagreement is not about whether mouthwash affects bacteria. It does. It is about how much that matters for a healthy person rinsing every day. The 2023 review proposed that the ideal mouthwash would fight disease while keeping the microbiome balanced, and said it remains uncertain which product fits that ideal.

Four common mistakes

Mistake 1. Treating all mouthwash as one thing

The studies above tested specific active ingredients. A result for chlorhexidine is not automatically a result for every bottle on the shelf.

Mistake 2. Rinsing right after brushing

The NHS advises against it because it washes away the concentrated fluoride your toothpaste left on your teeth. It suggests a different time, such as after lunch.

Mistake 3. Using a rinse instead of brushing

The ADA is clear that mouthwash is not a replacement for daily brushing and flossing.

Mistake 4. Rinsing away bad breath that will not quit

For bad breath, the NHS suggests trying an antibacterial mouthwash or toothpaste. But if it does not go away after a few weeks of looking after your mouth, the NHS advises seeing a dentist.

What to do with this

If a dentist recommended a rinse, use it as they advised. The ADA’s guidance is to talk to your dentist about whether you need a mouthwash and which kind. This article is not a reason to ignore that.

If you rinse out of habit, check the active ingredient on the label. That tells you which of the studies above, if any, actually applies to your bottle.

Frequently asked questions

Does mouthwash raise blood pressure?

In a 2013 study, one week of chlorhexidine raised blood pressure by roughly 2 to 3.5 mmHg in 19 healthy people. A 2020 study of 36 people saw systolic pressure rise, but not significantly. These studies tested chlorhexidine only, so they do not show what other rinses do.

Does mouthwash cause diabetes?

One observational study linked using mouthwash at least twice a day to a higher risk of prediabetes and diabetes. Observational studies cannot prove cause and effect.

Is alcohol-free mouthwash better for the oral microbiome?

The studies in this article did not compare alcohol-based and alcohol-free rinses. The 2024 study tested Listerine Cool Mint, which the journal’s publisher described as alcohol-based; its US label lists alcohol among the inactive ingredients.

Should I stop using mouthwash?

None of these studies tells healthy people to stop. The ADA advises talking to your dentist about whether you need a mouthwash and which kind. If a clinician recommended one, follow their guidance.

What about probiotic mouthwashes?

None of the studies in this article tested probiotic rinses. A future article here will look at what that research shows.

Watch the video version

Prefer to watch? The same breakdown, with every study on screen, is on The Vera Report YouTube channel. Next up: what the oral microbiome actually is.

Sources

  1. Kapil V et al. Physiological role for nitrate-reducing oral bacteria in blood pressure control. Free Radical Biology and Medicine, 2013;55:93-100. Link
  2. Bescos R et al. Effects of chlorhexidine mouthwash on the oral microbiome. Scientific Reports, 2020;10:5254. Link
  3. Laumen JGE et al. The effect of daily usage of Listerine Cool Mint mouthwash on the oropharyngeal microbiome: a substudy of the PReGo trial. Journal of Medical Microbiology, 2024;73(6). Link
  4. Joshipura KJ et al. Over-the-counter mouthwash use and risk of pre-diabetes/diabetes. Nitric Oxide, 2017;71:14-20. Link
  5. James P et al. Chlorhexidine mouthrinse as an adjunctive treatment for gingival health. Cochrane Database of Systematic Reviews, 2017. Link
  6. Brookes Z et al. Mouthwash effects on the oral microbiome: are they good, bad, or balanced? International Dental Journal, 2023;73(Suppl 2):S74-S81. Link
  7. Doel JJ et al. Evaluation of bacterial nitrate reduction in the human oral cavity. European Journal of Oral Sciences, 2005;113(1):14-19. Link
  8. McDonagh STJ. Factors influencing the effects of dietary nitrate supplementation on nitric oxide biomarkers and blood pressure. PhD thesis, University of Exeter, 2018. Link
  9. University of Liverpool: Viewpoint, Will your mouthwash kill you? (2014) Link
  10. Microbiology Society: Commonly used alcohol-based mouthwash brand disrupts the balance of your oral microbiome, scientists say (2024) Link
  11. NIHR Evidence: Chlorhexidine mouthwash is useful short-term for people with mild gum disease (2017) Link
  12. American Dental Association, MouthHealthy: Mouthwash. Link
  13. The Ohio State University Wexner Medical Center: Does mouthwash kill the mouth's healthy bacteria? Link
  14. NHS: How to keep your teeth clean. Link
  15. NHS: Bad breath. Link
  16. Listerine Cool Mint Antiseptic, US Drug Facts label (active ingredients: eucalyptol, menthol, methyl salicylate, thymol). Link

About the author. I am not a doctor or a researcher. I read supplement and oral care labels carefully and check claims against published research. If something here is wrong, tell me and I will correct it.

Medical disclaimer. This article is for general information only and is not medical advice. Nothing here is intended to diagnose, treat, cure or prevent any disease. Talk to a dentist or physician about your own situation before changing your oral care routine or starting any supplement.

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