Pathophysiological Insights Into the Oral–Brain Axis: Evidence‐Based Mechanisms Connecting Periodontitis and Alzheimer's Disease

ABSTRACT The bidirectional association between Alzheimer's disease ( AD ) and periodontitis (PD) has been recently demonstrated, indicating how the oral–brain axis connects the two conditions. Chronic oral inflammation caused by PD is accompanied by biological immunological responses, unchecked inflammation, and the spread of periodontal bacteria, all contributing to nervous system inflammation and AD pathogenesis. There are two primary pathways: (a) Inflammatory cascades, in which pro‐inflammatory cytokines, such as IL‐1β and TNF‐α, originating from periodontal lesions, cause inflammation in the brain region of the head, resulting in tau hyperphosphorylation, amyloid beta (Aβ) accumulation, and disruption of the blood–brain barrier; (b) Microbial involvement, in which oral pathogens, such as Porphyromonas gingivalis ( P. gingivalis ), can enter the bloodstream, enter the trigeminal nerve, and activate microglia. The fact that AD patients are known to experience greater periodontal disease than others, together with additional research maintaining the connection between oral dysbiosis and neurodegeneration, further supports these pathways. In older patients, the collapse of the blood–brain barrier exacerbates inhibitor breaches, allowing poisons and microorganisms to enter, increasing the formation of Aβ and neurotoxicity. Conversely, periodontal infections may exacerbate AD over time by causing a loss in peri‐oral neglect (cognitive decline) and self‐oral care (hygiene). To clarify the directed causative links that therapeutic approaches seek to resolve, rather than an attributable association, systematic reviews help interdisciplinary approaches focus on the integral integration of oral healthcare into AD preventive policies built around proactive AD management systems and longitudinal research studies. This evidence synthesis sets the oral–brain interaction as an axis of critically heightened focus for investigating AD pathogenesis, maximally shifting the paradigm for proactive intervention and tailored care models.