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Case in point:

No other alcohol-free mouthwash has been proven more effective at killing bad breath germs. Contains signature 4 Essential Oils found in LISTERINE® Antiseptic for bad breath protection.

How many germs can it possibly kill with essential oils and no alcohol?

Even the mouthwash with alcohol lists the essential oils like eucalyptol and menthol as active ingredients, and relegates alcohol to inactive ingredients, which raises the question why the alcohol is even needed. Also, the alcohol is usually around 20%. In contrast, hand sanitizers have over 60% alcohol, but mouthwash can't use over 40% alcohol because it raises the risk of oral cancer.

  • Technically speaking eucalyptol and menthol are alcohols... so the product is not alcohol free! – nico Jul 8 '13 at 18:50
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    That's cute wording: "No other alcohol-free mouthwash has been proven more effective." That would still be true if all of them had been proven to be completely ineffective! – Nate Eldredge Oct 18 '13 at 22:27
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According them them, their 4 essential oils are

  • Eucalyptol
  • Menthol
  • Methyl salicylate
  • Thymol

Menthol, Thymol and Eucalyptol do have known antibacterial effects. I'm having difficulty finding anything specifically on Methyl salicylate (wintergreen oil), but their claims of efficacy against plaque seem plausible. Additionally, a Japanese study comparing Listerine's effectiveness with and without the essential oils did find that they have a significant effect.

Additionally, looking at the ingredients list:

Water, sorbitol solution, propylene glycol, sodium lauryl sulfate, benzoic acid, sodium benzoate, poloxamer 407, sodium saccharin, flavor, sucralose, FD&C green no. 3

We find 3 other ingredients that may also contribute to the anti-plaque action. Sodium lauryl sulfate is a microbiocide, Benzioc acid is an antiseptic, and sodium benzoate is a bacteriostatic. Depending on the concentrations, these may contribute further.

4

EO's can definately kill bacteria, however the other complicating factor related to alcohol is the biofilm.

Penetrating the plaque biofilm: impact of essential oil mouthwash.

This suggests that an effective mouthwash must also penetrate the plaque biofilm. Two studies have demonstrated the ability of an EO mouthwash to penetrate the plaque biofilm.

Oil Essential Mouthwashes Antibacterial Activity against Aggregatibacter actinomycetemcomitans: A Comparison between Antibiofilm and Antiplanktonic Effects

In conclusion, the alcoholic mouthwash appears to have a better biofilm inhibition than its antiplanktonic activity while the nonalcoholic product demonstrates an opposite effect with a better antiplanktonic behavior.

From the results, the huge difference between mouthwashes activity against Aa in its planktonic and biofilm forms clearly appears. Firstly, alcoholic products behave in a different way from the other two mouthwashes so that, while they have the worst effect against PG, they show at the same time the best antibiofilm activity at high dilution. In accordance with the literature, essential oils with alcohol have the best antibiofilm activity [23–25]. On the base of these results, it seems that an alcoholic mouthwash could be a good choice in preventing plaque formation after periodontal therapies.

In contrast, essential oil nonalcoholic mouthwashes show a better antiplanktonic rather than antibiofilm activity. In this case, it could be considered a good choice in order to prevent bacterial systemic dissemination. In particular, they could be useful for treating specific anatomic areas with a high risk of bacteraemia.

  • It would be useful for you to include a summary of the key points in the quotes. This makes for better answers. – matt_black Jan 16 '16 at 13:21
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As you noted, alcohol is not listed as an active ingredient. Likewise, J&J does not claim that the alcohol helps kill germs, at least not in the link provided. Furthermore, they do state that "[in] LISTERINE® Antiseptic products, alcohol is an inactive ingredient that is used as a solvent, and a delivery vehicle to carry the fixed combination of 4 ESSENTIAL Oils." Thus, assuming their claim is true, the question is how the alternative solvent compares to alcohol in delivering the oils.

This study concluded that Listerine Zero was more effective in reducing plaque and gingivitis than the negative control 5% hydroalcohol mouthrinse (the abstract doesn't specify brand). Note that the study was conducted by J&J, the manufacturer of Listerine. Also, it doesn't answer the more important question: how Listerine Zero compares to normal Listerine.

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