Chapter 6: The Problem with Scorched Earth

In which we examine the evidence that antiseptic mouthwashes may be causing harm, including a surprising connection to cardiovascular health

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I want to be careful here, because I'm about to criticize products that millions of people use daily, that are recommended by many dental professionals, and that do have some legitimate therapeutic applications. I'm not trying to tell you that everyone who has ever used Listerine has been poisoning themselves. That would be alarmist and inaccurate.

But I am going to present evidence that regular, long-term use of antiseptic mouthwashes for daily maintenance—as opposed to therapeutic use for specific conditions—may not be doing what you think it's doing. And it may be doing some things you would never have expected.

Let's look at the data.

The Efficacy Question

First, do antiseptic mouthwashes actually prevent oral disease in healthy people who are already brushing and flossing?

This turns out to be a surprisingly difficult question to answer definitively, because most studies are short-term (weeks to months) and measure intermediate outcomes like plaque indices and gingival bleeding rather than long-term outcomes like cavity development or tooth loss.

What we can say:

Chlorhexidine is genuinely effective at reducing plaque and gingivitis over the short term. Multiple systematic reviews confirm this.¹ However, chlorhexidine has significant side effects—tooth staining, taste disturbance, increased calculus formation—that make it unsuitable for indefinite daily use. It's typically recommended for short-term therapeutic courses (post-surgical, during periodontal treatment) rather than ongoing maintenance.

Alcohol-based mouthwashes (like traditional Listerine) show more modest effects. A 2009 Cochrane review found that chlorhexidine mouthwash reduced plaque by about 33% and gingivitis by about 26% compared to placebo or no treatment.² But the studies in that review were short-term, and the clinical significance of a 26% reduction in a gingivitis index isn't entirely clear.

Cetylpyridinium chloride (CPC) mouthwashes have weaker antimicrobial activity than chlorhexidine. Evidence of meaningful clinical benefit is less robust.

Here's the critical gap: very few studies have examined whether long-term antiseptic mouthwash use prevents actual dental disease—cavities, periodontal destruction, tooth loss—in people who are already performing mechanical oral hygiene. We have abundant evidence that antiseptics kill bacteria in the short term. We have limited evidence that this translates to better long-term oral health outcomes in the general population.

The assumption that killing bacteria must help has been doing a lot of work.

The Microbiome Disruption Problem

Even if antiseptic mouthwashes don't cause direct harm, they may cause indirect harm by disrupting the oral ecosystem in ways that don't favor health.

A 2020 study published in Scientific Reports examined the oral microbiome composition of regular antiseptic mouthwash users versus non-users.³ The researchers found significant differences in community composition, with mouthwash users showing:

• Reduced bacterial diversity
• Altered proportions of major bacterial taxa
• Specifically, reduced abundance of certain nitrate-reducing species

These changes persisted during the study period and appeared to stabilize into a different community configuration—not a temporary disruption that recovered, but a sustained ecological shift.

Now, an altered microbiome isn't necessarily a harmful microbiome. But given what we know about the benefits of certain oral bacteria (alkali production, pathogen competition, nitrate reduction), a less diverse community with fewer of these beneficial species seems unlikely to be an improvement.

The "kill 99.9% of germs" claim that appears on so many products is technically true for the moments after use. But that 0.1% that survives, plus recolonization from surfaces the mouthwash didn't reach (deep in biofilms, in crypts of the tongue), means the mouth repopulates within hours. The question is what repopulates—and there's no guarantee it's a healthier community than what was there before.

The Blood Pressure Connection

This is the finding that, for me, changed how I think about antiseptic mouthwash use.

Remember the nitrate-reducing bacteria we discussed earlier? The ones that convert dietary nitrate to nitrite, which then becomes nitric oxide—a vasodilator that lowers blood pressure?

Those bacteria live in your mouth. When you use antiseptic mouthwash, you kill them along with everything else.

A 2020 study in Free Radical Biology and Medicine enrolled 27 adults and had them use antiseptic mouthwash (0.12% chlorhexidine) twice daily for one week, with measurements of blood pressure, oral nitrate-reducing bacteria, and plasma nitrite levels before and after.⁴

The results:

• Systolic blood pressure increased by approximately 2-3.5 mmHg during the mouthwash period
• Oral nitrate-reducing bacteria were substantially reduced
• Plasma nitrite levels fell, indicating impaired nitrate-to-nitrite conversion
• These effects were observed regardless of baseline blood pressure

A 2-3 mmHg increase in systolic blood pressure might not sound like much. But at a population level, such an increase would translate to meaningful increases in cardiovascular events. And this was after just one week of use.

Other studies have corroborated this finding:

• A 2017 study found that antiseptic mouthwash blunted the blood pressure-lowering effects of exercise, which also works partly through nitric oxide pathways⁵
• A large observational study found an association between regular antiseptic mouthwash use and increased risk of developing hypertension over time

The mechanism is clear: antiseptic mouthwashes disrupt the entero-salivary nitrate-nitrite-nitric oxide pathway by killing the oral bacteria that perform the first step.

This is not a theoretical concern about possible disruption of some abstract ecosystem. This is a measurable effect on a parameter—blood pressure—that directly affects cardiovascular health.

I want to be clear about the limitations: these studies don't prove that mouthwash causes heart attacks. The blood pressure effects are modest (though meaningful at population scale). Causality in the observational studies isn't definitively established. More research is needed.

But consider: if a new medication were found to raise blood pressure by 2-3 mmHg in everyone who took it, that would be considered a significant adverse effect. Why should a daily oral hygiene product be held to a different standard?

The Alcohol Controversy

Traditional antiseptic mouthwashes contain substantial amounts of alcohol—up to 27% in some formulations, higher than wine. This alcohol serves as a solvent for the active ingredients, as a preservative, and as an antimicrobial agent in its own right.

There has been long-running controversy about whether alcohol-containing mouthwashes increase the risk of oral cancer.

The mechanism would be plausible: alcohol is a known carcinogen, the oral mucosa is directly exposed, and some people use these products multiple times daily for years or decades. Acetaldehyde, the first metabolite of alcohol and a confirmed carcinogen, is produced in the mouth from alcohol by oral bacteria.

The evidence is genuinely mixed:

• A 2009 review and meta-analysis found a "statistically significant association" between mouthwash use and oral cancer⁶
• However, subsequent reviews questioned the methodology, noting that many studies didn't adequately control for smoking and alcohol drinking—the major risk factors for oral cancer
• A 2016 position paper from the Australian Dental Association concluded that "there is insufficient evidence that alcohol-containing mouthrinses contribute to oral cancer risk in non-smokers and non-drinkers"⁷

My read of the evidence: for non-smokers who don't drink heavily, the oral cancer risk from alcohol-containing mouthwash is probably small or nonexistent. But "probably small" isn't the same as "none," and the uncertainty should give pause.

More importantly, whatever the cancer question, the alcohol is contributing to the antimicrobial effect that may be causing microbiome disruption and the documented blood pressure effects. The alcohol isn't just a neutral solvent; it's part of the problem.

Local Tissue Effects

Beyond systemic effects, antiseptic mouthwashes can cause local irritation:

• Mucosal irritation and desquamation: High-alcohol mouthwashes can damage the oral mucosa, especially with frequent use
• Dry mouth: Alcohol is a drying agent; paradoxically, a product meant to improve oral health may reduce the saliva that provides natural protection
• Taste disturbance: Particularly with chlorhexidine, but to some degree with other antiseptics
• Tooth staining: Chlorhexidine is notorious for this, but other agents can also contribute
• Altered taste sensation: Some users report persistent taste changes even after discontinuing use

These effects are usually described as "minor" or "cosmetic," but they're not trivial for people who experience them. And the dry mouth effect is particularly counterproductive given how important saliva is for oral health.

When Antiseptics Are Appropriate

I've been making the case against routine antiseptic mouthwash use, but I want to be clear: there are legitimate therapeutic applications.

Post-surgical use: After oral surgery, including extractions, the risk of infection is elevated and normal oral hygiene may be difficult or impossible. Short-term chlorhexidine use during the healing period is standard of care.

Active periodontal disease: When periodontitis has been diagnosed and professional treatment is ongoing, antiseptic rinses may help control bacterial populations while the inflammatory condition is being addressed.

Patients at very high caries risk: In some cases—severe dry mouth, high S. mutans counts, history of rampant decay—more aggressive antimicrobial approaches may be justified.

Immunocompromised patients: Those with reduced immune function may benefit from antiseptic protection that healthy individuals don't need.

Pre-procedural rinses: A brief rinse before dental procedures reduces bacterial aerosols and may reduce post-operative infection risk.

The pattern here is that antiseptics are useful for specific therapeutic situations, typically short-term, where the benefits clearly outweigh the disruption costs. What's questionable is extending these products to daily maintenance in healthy individuals.

The Marketing Reality

Why do people use antiseptic mouthwash daily despite the limited evidence of benefit and the emerging evidence of harm?

Partly because of marketing. Listerine's famous campaigns—"Kills Germs That Cause Bad Breath," "Kills 99.9% of Germs"—have been extraordinarily successful at equating antibacterial action with oral health. The burning sensation, which might otherwise be perceived as irritation, has been reframed as evidence that "it's working."

Partly because of authority. When dental professionals recommend mouthwash without specifying that they mean therapeutic use, patients reasonably interpret this as an endorsement of daily use.

Partly because of intuition. The germ theory is so deeply embedded in our cultural understanding that "killing germs" seems obviously good. The idea that killing germs might be harmful—that some germs are helpful—requires a more sophisticated understanding than most people have been given.

And partly because alternatives haven't been clearly presented. If you're told that brushing and flossing aren't enough, and the only additional measure you know about is antiseptic mouthwash, that's what you'll reach for.

The Bottom Line

Let me summarize what the evidence supports:

1. Antiseptic mouthwashes do kill bacteria in the short term. This is not in question.

2. Whether this killing prevents oral disease in healthy people who brush and floss is less clear than commonly assumed.

3. Antiseptic mouthwashes disrupt the oral microbiome in ways that may not favor health, including reduction of beneficial species.

4. Antiseptic mouthwashes impair nitrate reduction and measurably raise blood pressure through disruption of the nitric oxide pathway.

5. Local effects including irritation, dryness, and taste disturbance are documented.

6. Specific therapeutic applications (post-surgical, active periodontal disease, high-risk patients) remain appropriate.

If you're a healthy person using antiseptic mouthwash twice daily because you think it's helping your oral health, the evidence suggests you should reconsider. At best, you're spending money on a product of minimal benefit. At worst, you're raising your blood pressure, disrupting beneficial bacteria, and potentially increasing long-term risks we haven't yet fully characterized.

There are better approaches. They're older approaches, in many cases. They work with your mouth's ecology rather than against it.

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Which brings us to the paradigm shift that's underway—from warfare to stewardship, from scorched earth to ecosystem management.

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1. James, P., et al. (2017). Chlorhexidine mouthrinse as an adjunctive treatment for gingival health. Cochrane Database of Systematic Reviews, 3, CD008676. ↩

2. Gunsolley, J. C. (2006). A meta-analysis of six-month studies of antiplaque and antigingivitis agents. Journal of the American Dental Association, 137(12), 1649-1657. ↩

3. Bescos, R., et al. (2020). Effects of chlorhexidine mouthwash on the oral microbiome. Scientific Reports, 10(1), 5254. ↩

4. Tribble, G. D., et al. (2019). Frequency of tongue cleaning impacts the human tongue microbiome composition and enterosalivary circulation of nitrate. Frontiers in Cellular and Infection Microbiology, 9, 39. ↩

5. Cutler, C., et al. (2019). Post-exercise hypotension and skeletal muscle oxygenation is regulated by nitrate-reducing activity of oral bacteria. Free Radical Biology and Medicine, 143, 252-259. ↩

6. McCullough, M. J., & Farah, C. S. (2008). The role of alcohol in oral carcinogenesis with particular reference to alcohol-containing mouthwashes. Australian Dental Journal, 53(4), 302-305. ↩

7. Australian Dental Association. (2009). Policy Statement: Alcohol-containing mouthwashes and oral cancer. ↩