Emerging evidence suggests that poor oral health is associated with Alzheimer’s disease (AD), but there is a lack of large-scale clinical studies demonstrating this link. The increased risk of dementia is potentially due to the spread of bacteria and inflammatory molecules from the mouth to the brain. There is also a bidirectional relationship between oral and cognitive health, in that people with dementia may have difficulty maintaining good oral hygiene, which can lead to increased risk of gum disease.

In an article published in Brain Sciences in 2023, Kulkarni et al. conducted a study to generate large-scale data from the TriNetX database to determine risk associations and assess survival between poor oral health and AD in people 60 years and older. Their goal was to compare the prevalence of AD between individuals with and without poor oral health and determine which factors contribute to AD risk and survival. They found that patients with poor oral health demonstrated a 2.363-fold increased risk of AD compared to those in the normal oral health cohort. They also found that poor oral health was a risk factor for AD independent of age, gender, and laboratory measures. An association with a higher risk of AD was found with dental caries, periodontal diseases, gingivitis, and other diseases related to tooth loss and diseases of the lip and oral mucosa. They concluded that maintaining or improving healthy oral hygiene and/or oral health may significantly reduce the risk of AD.

A 2024 article by Liu et al., in Critical Reviews in Microbiology, suggested that the bacteria P. gingivalis may be a cause of AD. The bacteria produce enzymes called gingipains which kill nerve cells. P.gingivalis is also a major cause of periodontitis.  People who have died from AD have large amounts of gingipains, and P.gingivalis DNA has been found in their brains. P.gingivalis carries vesicles that produce proteins and enzymes which stimulate inflammation, and which can enter the brain as they can cross the blood-brain barrier (BBB). Once in the brain they accelerate the production of amyloid and abnormal tau proteins which can lead to neuronal cell death. These changes occur before the diagnosis of AD, so P.gingivalis is thought to be a possible cause of Alzheimer’s dementia.

A study reported in the 2020 Journal of Alzheimer’s Disease (Beydoun et al.) suggests that the bacteria responsible for gum disease is associated with Alzheimer’s disease. Though large studies have not been conducted to confirm this, it is believed that bacteria and the inflammatory molecules they make can travel from infections in the mouth through the bloodstream to the brain.  In this study, researchers examined whether gum disease and infections with oral bacteria were linked to dementia diagnoses and deaths. The team compared different age groups at baseline, with up to 26 years of follow-up, for more than 6,000 participants. The analysis revealed that older adults with signs of gum disease and mouth infections at baseline were more likely to develop AD during the study period. The findings suggest that oral infection preceded the diagnosis of dementia. Also, having dementia makes it more likely that an individual will not be able to brush and floss effectively, which increases the likelihood of such infections and gum disease. It should, however, be noted that population studies can show association but not causality.

A study by L’Heureux et al., published in 2025 in PNAS Nexus, found that individuals with higher levels of Porphyromonas, a key pathogen in chronic periodontitis, were more likely to experience memory issues. Additionally, the bacteria Prevotella, commonly associated with periodontal disease, was linked to the presence of the APOE4 gene, a known risk factor for Alzheimer’s disease. The principal findings of this study were that a greater prevalence of oral P. intermedia is linked to elevated genetic risk for dementia (APOE4 genotype) in individuals with MCI prior to dementia diagnosis, and that interventions that promote the oral Neisseria–Haemophilus and suppress Prevotella-dominated modules have potential for delaying cognitive decline.

Considering the evidence discussed above, it appears that better oral health may lead to a reduction in the risk of AD. It would be wise for people to be more proactive regarding their oral health, and regularly brush, floss, use mouthwash and visit a dental hygienist.