<jats:sec> <jats:title>Introduction</jats:title> <jats:p>Severe periodontitis is a major global health concern, yet the impact of switching from combustible cigarettes to dual use of combustible cigarettes and heated tobacco products on periodontal status remains unclear. This study aimed to examine whether changes in smoking status influence the risk of severe periodontitis among Japanese adults.</jats:p> </jats:sec> <jats:sec> <jats:title>Methods</jats:title> <jats:p>We conducted a longitudinal analysis using panel data from the Japan COVID-19 and Society Internet Survey. Participants were daily combustible cigarette smokers with periodontitis in 2021 and were followed through 2023. Smoking trajectories between 2021 and 2022 were categorized as: (1) continuous combustible cigarette smokers, (2) dual users (heated tobacco product-dominant), (3) dual users (combustible cigarette-dominant), and (4) quitters/reduced smokers. Severe periodontitis in 2023 was defined using validated self-reported criteria. Modified Poisson regression with robust variance was applied to estimate risk ratios and 95% confidence intervals (95% CIs), adjusting for potential confounders (age, sex, educational attainment, occupational status, marital status, equivalent income, alcohol consumption, diabetes mellitus status, periodontal pain, and smoking frequency).</jats:p> </jats:sec> <jats:sec> <jats:title>Results</jats:title> <jats:p>A total of 193 participants (mean age 57.6 years; men 67.9%) were analyzed. Over 2 years, 61.1% developed severe periodontitis. Compared to continuous combustible cigarette smokers, the adjusted risk of severe periodontitis was 1.19 times (95% CI: 0.67–2.11), 1.41 times (95% CI: 1.04–1.91), and 0.99 times (95% CI: 0.76–1.28) higher for dual users (heated tobacco product-dominant), dual users (combustible cigarette-dominant), and quitters/reduced smokers, respectively. Sensitivity analyses using inverse probability weighting yielded consistent findings.</jats:p> </jats:sec> <jats:sec> <jats:title>Conclusion</jats:title> <jats:p>Switching from daily combustible cigarette smoking to dual use with heated tobacco products may increase the risk of severe periodontitis. Combining combustible cigarettes with heated tobacco products, regardless of the frequency of combustible cigarette smoke, should not be considered a harm-reduction strategy in dental and public health practice.</jats:p> </jats:sec>

<jats:sec id="ab-aoi250075-4"> <jats:title>Importance</jats:title> <jats:p>In Japan, the public assistance (PA) system for low-income individuals provides medical aid that covers all health care costs (no cost sharing). However, little is known about how health care service utilization changes after PA certification.</jats:p> </jats:sec> <jats:sec id="ab-aoi250075-5"> <jats:title>Objective</jats:title> <jats:p>To examine the trajectories of outpatient medical and dental care utilization among PA recipients before and after PA certification.</jats:p> </jats:sec> <jats:sec id="ab-aoi250075-6"> <jats:title>Design, Setting, and Participants</jats:title> <jats:p>Longitudinal data from a municipality in Japan participating in the Longevity Improvement Fair Evidence Study from 2017 to 2022 were used. Generalized estimating equations were used to calculate absolute differences and relative ratios in health care service utilization before and after PA certification. An interrupted time-series analysis (ITSA) was performed to examine the trends in health care service use based on monthly panel data. The elasticity of each health care service was also calculated. The participants were individuals who switched from National Health Insurance to PA and were 20 years and older. The analysis was performed in 2024.</jats:p> </jats:sec> <jats:sec id="ab-aoi250075-7"> <jats:title>Exposure</jats:title> <jats:p>PA certification.</jats:p> </jats:sec> <jats:sec id="ab-aoi250075-8"> <jats:title>Main Outcomes and Measures</jats:title> <jats:p>Outpatient medical and dental care expenditures, visits, and unit costs per month from 1 year before to 1 year after PA certification.</jats:p> </jats:sec> <jats:sec id="ab-aoi250075-9"> <jats:title>Results</jats:title> <jats:p>Overall, 2893 study participants were analyzed (mean [SD] age, 54.2 [15.3] years; 1501 women [51.9%]). For outpatient medical care, the mean (SD) expenditures per month before and after PA certification were \16 565 (\53 159) and \22 442 (\53 443), respectively (converting to US dollars using the rate of \100 = $0.68 based on 2025 rates). For outpatient dental care, the mean (SD) expenditures per month before and after PA certification were \1727 (\3726) and \3978 (\6894), respectively. The ITSA showed significant increases in outpatient expenditure immediately after PA certification: \2681 (95% CI, 317-5046) and \2330 (95% CI, 1896-2764) in medical and dental care, respectively.</jats:p> </jats:sec> <jats:sec id="ab-aoi250075-10"> <jats:title>Conclusions and Relevance</jats:title> <jats:p>This study found that eliminating cost sharing under a PA system was associated with increased outpatient health service use, particularly for dental care. These findings highlight financial barriers to care among low-income populations and suggest that no cost-sharing policies can improve access but may require policy measures to address overuse.</jats:p> </jats:sec>