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Background and Objectives: Diabetes mellitus is a major systemic modifier of oral infection, periodontal inflammation, and tissue healing. Persistent hyperglycemia may intensify oxidative stress, immune dysfunction, dysbiotic biofilm activity, and advanced glycation end-product/receptor signaling, thereby reducing the predictability of periapical and periodontal healing. In patients with endo-periodontal lesions, these mechanisms may interact with pulpal infection and periodontal breakdown, complicating diagnosis, prognosis, and therapeutic sequencing. This review summarizes current evidence on diabetes, glycated hemoglobin (HbA1c), and endo-periodontal outcomes and proposes an HbA1c-guided clinical framework for risk stratification and treatment planning. Materials and Methods: A structured narrative review with an expert-informed clinical framework was conducted using the literature published between January 2016 and December 2025. Eligible sources included consensus reports, clinical practice guidelines, systematic reviews, meta-analyses, umbrella reviews, randomized and non-randomized clinical studies, observational studies, and selected mechanistic studies or contemporary narrative reviews with direct relevance to the framework. Case reports, animal studies, and in vitro investigations without direct relevance to the clinical framework were excluded from the main synthesis. Results: Diabetes is associated with increased periodontal severity, impaired periodontal treatment response, a higher prevalence of radiolucent periapical lesions in root-filled teeth, delayed periapical healing, and increased non-retention of endodontically treated teeth. Periodontal therapy is safe in diabetic patients and may be associated with modest reductions in HbA1c, especially when baseline metabolic control is poor. Conclusions: HbA1c should complement, not replace, pulpal testing, periodontal charting, and radiographic assessment. The proposed framework prioritizes endodontic infection control when pulpal necrosis or active intracanal infection is present, adapts periodontal intervention to metabolic risk, and postpones elective surgical or regenerative procedures in poorly controlled diabetes until medical stabilization is achieved.