Background/Objectives: The Charlson Comorbidity Index (CCI), which focuses on 19 comorbid diseases and conditions, has been widely used as a valid predictor of mortality. This study aimed to comprehensively examine the prevalence of nearly all comorbidities and concomitant medications according to CCI classification (CCI = 0 and CCI ≥ 4) and age group (aged 40–44 and 70–74 years) in middle-aged Japanese adults. Methods: The present study included 9,182,226 individuals who underwent health checkups from April 2018 to March 2019. A total of 15,916 cases of diagnosed diseases and conditions, including communicable diseases; diseases of the eye, ear, skin, and musculoskeletal system; and psychiatric disorders, were investigated alongside 16,886 prescribed medications. Results: The prevalence of allergic rhinitis was ranked among the leading comorbidities in all age groups and CCI categories. Individuals with a CCI ≥ 4 in the 40–44 age group showed a higher prevalence of cardiometabolic diseases such as hypertension, diabetes, and dyslipidemia compared with individuals with a CCI = 0 in the 70–74 age group. Furthermore, individuals with a CCI ≥ 4 in the 40–44 age group also had a higher prevalence of communicable diseases, gastrointestinal symptoms, iron deficiency anemia, and psychiatric disorders compared with individuals with a CCI = 0 in the 70–74 age group. The ranking for prescribed medications was essentially the same between age groups, but was found to differ between CCI categories. Conclusions: This study identified overlooked comorbidities and concomitant medications that are not accounted for in the 19 conditions included in the CCI, which may be important prognostic factors in determining mortality. Although patients with more comorbidities were found to be more frequently diagnosed with cardiometabolic diseases regardless of their age, the presence of pharmacotherapy may be dependent on age.
Sekine et al. (2026) studied this question.