Dentists recommend patients with halitosis (bad breath) and periodontal disease to use mouth washes to freshen up their breath and control the proliferation of oral bacteria. But some formulations have been shown to cause some unintended side effects with a longer duration of use. Amid some allegations against popular mouthwash brands like Listerine, it’s important to examine what current scientific evidence and research actually reveal.
Listerine is a widely marketed antiseptic mouthwash, available over the counter, and used as part of oral hygiene routines. It contains active ingredients such as essential oils (e.g. thymol, eucalyptol, menthol) and, in many formulations, alcohol as a solvent and carrier. Alcohol content in mouthwashes may reach up to ~20–27 % by volume. New formulations also offer alcohol free options. It is used to reduce microbial load in the oral cavity, reach areas not fully addressed by brushing or flossing, and help control plaque, gingivitis, and bad breath.
Clinical studies and reviews report several adverse effects associated with routine mouthwash use:
Teeth staining is the most frequently cited side effect in longer trials due to chlorhexidine in most mouthwashes. Dentists recommend discontinuing using the product after a month for 2 weeks and resume if needed.
Mucosal irraitation may cause burning effect, altered taste sensation, and abnormal oral sensations have been documented in users. 1
Cytotoxic effects: In vitro studies on human gingival epithelial progenitor cells found that certain mouthwash components (e.g. hydrogen peroxide, chlorhexidine, chlorine dioxide, cetylpyridinium chloride) at even low concentrations can impair cell viability or induce apoptosis depending on exposure time and concentration which might alter the organic oral flora and affect the equilibrium.2
Disruption of the oral microbiome: A randomized controlled trial of Listerine Cool Mint over three months showed a shift in oral bacterial composition, with increased abundance of Fusobacterium nucleatum and Streptococcus anginosus, species previously linked to periodontal disease and certain cancers, which has been the basis of the class action lawsuit filed against them.3
Risk of gum disease and microbiome imbalance: Critics argue that alcohol-based mouthwashes may kill beneficial bacteria and allow proliferation of harmful species.
The possibility of a link between long-term mouthwash use and head and neck cancers has been the subject of several research and pooled analyses. A particular one of 12 case-control studies conducted by the INHANCE consortium found that individuals who had used mouthwash for more than 35 years showed a modestly elevated odds ratio (OR) of 1.15 (95% CI: 1.01–1.30) for developing head and neck cancer compared to never-users. Similarly, those who reported using mouthwash more than once daily demonstrated a slightly higher risk, with an OR of 1.31 (95% CI: 1.09–1.58). The same analysis estimated the risk for oral cavity cancer among ever-users at OR 1.11 (95% CI: 1.00–1.23).4
However, it is important to note that a significant proportion of regular mouthwash users are also smokers who may use these products to mask tobacco-related odor. Therefore, the potential confounding effect of smoking—along with alcohol consumption—should be carefully considered when evaluating any association between mouthwash use and cancer risk.
Other meta-analyses and systematic reviews have reported weak or statistically insignificant associations after accounting for confounding factors such as tobacco and alcohol consumption. A recent systematic review published in Cureus emphasized that while certain studies suggest a possible increase in oral cancer risk among frequent mouthwash users, methodological variations and the influence of concurrent risk factors make it difficult to draw definitive conclusions. 5Despite these concerns, alcohol-containing mouthwash formulations continue to be widely used in clinical practice. Reviews have consistently indicated that the existing epidemiological evidence does not establish a firm causal link between mouthwash use and cancer risk.
In 2024, a class-action lawsuit was filed against Johnson & Johnson and Kenvue over the Listerine Cool Mint formulation. The complaint alleges that the product’s labeling fails to warn consumers that regular use could promote proliferation of bacteria linked with cancer. Lawyer Angela Kim, Harvard Law School graduate, explains about this lawsuit in a popular Instagram reel which has been shared by Dr. Emeka Ajufo, a physiatrist.
The suit cites the above mentioned microbiome trial showing increased levels of F. nucleatum and S. anginosus after daily usage. The plaintiff claims that the consumer protection law mandates transparent labeling of potential risks. The manufacturers have responded by denying a causal relationship between their product and cancer, citing decades of safety data.
Conclusion
Listerine remains a widely used oral hygiene adjunct. While side effects are documented, and emerging research raises questions about long-term cancer risk and microbial imbalance, the evidence is not definitive. The class-action lawsuit reflects consumer concern and interest in labeling transparency. Until more conclusive data emerge, users and dentists may consider balancing benefits with potential risks, especially for long-term use or repeated daily use.
References
Tartaglia, Gianluca M., Santosh Kumar Tadakamadla, Stephen T. Connelly, Chiarella Sforza, and Conchita Martín. “Adverse Events Associated with Home Use of Mouthrinses: A Systematic Review.” Therapeutic Advances in Drug Safety (2019). PMID: 31579502. https://pubmed.ncbi.nlm.nih.gov/31579502/.
Kőhidai, Zsófia, Angéla Takács, Eszter Lajkó, Zoltán Géczi, Éva Pállinger, Orsolya Láng, and László Kőhidai. “The Effects of Mouthwashes in Human Gingiva Epithelial Progenitor (HGEPp) Cells.” Clinical Oral Investigations 26 (2022): 4559–4574. https://link.springer.com/article/10.1007/s00784-022-04422-z.
“The Effect of Daily Usage of Listerine Cool Mint Mouthwash on the Oropharyngeal Microbiome: A Substudy of the PReGo Trial.” Journal / Source (2025). PMID: 38833520. https://pubmed.ncbi.nlm.nih.gov/38833520/.
“Mouthwash Use and Oral Cancer Risk: A Pooled Analysis from the INHANCE Consortium.” PMC Article. PMC ID: 4752930. https://pmc.ncbi.nlm.nih.gov/articles/PMC4752930/.
“Does Mouthwash Use Affect Oral Cancer Risk? A Comprehensive Systematic Review and Meta-Analysis.” Cureus (2024). https://www.cureus.com/articles/324981-does-mouthwash-use-affect-oral-cancer-risk-a-comprehensive-systematic-review-and-meta-analysis.
