Periodontitis and Diabetes Mellitus: A Critical Narrative Review of Bidirectional Mechanisms, Causal Evidence and Clinical Management

Periodontitis and diabetes mellitus are prevalent chronic conditions with overlapping inflammatory, behavioural and social determinants. Their association is often described as bidirectional, but this expression can obscure important differences between the evidence that dysglycaemia contributes to periodontal destruction and the evidence that periodontal inflammation causes diabetes or its complications. This critical narrative review integrates clinical epidemiology, experimental mechanisms, genetic causal-inference studies, randomised treatment trials, professional guidelines and implementation research. Literature published from January 2000 to 27 August 2026 was considered through targeted scholarly and institutional searching, citation tracing and critical appraisal. Hyperglycaemia is a credible modifier of periodontal susceptibility and progression through advanced glycation signalling, altered innate immunity, oxidative stress and impaired tissue repair. Periodontitis is associated prospectively with incident type 2 diabetes, yet residual confounding, periodontal exposure misclassification and inconsistent Mendelian randomisation findings restrict causal interpretation. By contrast, moderate-certainty trial synthesis indicates that active subgingival periodontal treatment can modestly improve glycated haemoglobin among adults with diabetes and periodontitis, although the size and persistence of benefit vary and evidence for preventing renal, cardiovascular or mortality outcomes remains insufficient. The review distinguishes risk prediction from causal intervention, evaluates why trials disagree, and examines practical coordination of dental and diabetes care. The most defensible clinical position is to diagnose and manage periodontitis for established oral-health indications while incorporating periodontal assessment into person-centred diabetes care. Priority research needs include adequately powered, longer-term pragmatic trials, harmonised periodontal phenotyping, diverse prospective cohorts, improved genetic instruments and implementation evaluations that measure equitable access rather than referral alone.