This longitudinal study used panel data obtained from the Japan COVID-19 and Society Internet Survey (JACSIS; https://jacsis-study.jp/index.html). JACSIS was designed to explore information on lifestyle, health, social, and economic factors, including the impact of COVID-19, through internet-based questionnaires. The pre-baseline survey, conducted in 2021, was distributed to 35,719 panelists registered with a Japanese internet research company between September 27 and October 29, 2021. The 2022 survey followed from September 12 to October 19, and the 2023 survey was conducted between September 25 and November 17, 2023. This study included participants from the 2021 survey who were daily smokers of only CCs and had periodontitis. Individuals under 20 years or over 75 years, as well as those who selected “other” for their educational information, and those whose answers were considered unreliable responses (i.e., the participants answered they had all current chronic disease or drug use listed in the options), were excluded from the analysis.

The transition from periodontitis in the 2021 survey to severe periodontitis in the 2023 survey was used as the outcome variable in this study. First, periodontitis in the 2021 survey was evaluated using four questions: “Have your gums bled recently?”, “Do you think your gums have receded, making your teeth appear longer than before?”, “Have you ever been told at a dental clinic that you needed treatment for periodontitis or gums issues?”, and a question about smoking status. For the smoking status question, participants were asked whether they currently or previously smoked CCs. Participants were classified as having periodontitis if they answered “Yes” to three or four of these questions. This definition of screening in periodontitis has been reported in a previous study (19). In the current study, the target participants were daily CC smokers in the 2021 survey and were automatically categorized as “Yes” for the smoking status question. Therefore, periodontitis in the 2021 survey was defined for participants who answered “Yes” to at least two of the other three questions regarding periodontal status. Second, severe periodontitis in the 2023 survey was determined using the following four questions: “Do you think you might have periodontitis?”, “Have you ever had a loose tooth?”, “Have you ever been told by a dentist or dental hygienist that you have bone loss around your teeth?” and “Have you experienced gum bleeding in the last 3 months?” Participants were classified as having severe periodontitis if they answered “Yes” to two or more of these questions. This definition of severe periodontitis was validated in a prior study (20).

The change in smoking status from the 2021 survey to the 2022 survey was used as the exposure variable. Both surveys included the same question regarding the frequency and type of tobacco products participants smoked or used: “Have you currently smoked or used each tobacco product?” Participants selected one of the following responses: “I have never smoked or used.”, “I smoked or used more than once but have not used routinely.”, “I had smoked or used routinely before but quit recently.”, “I smoke or use sometimes.”, or “I smoke or use almost every day.” The tobacco products listed included CCs (hand-rolled tobacco and manufactured cigarettes) and HTPs, which are the predominant tobacco products used in Japan (21). Other tobacco or nicotine products were not included due to their very low prevalence. Additionally, participants were asked how many days they smoked CCs or used HTPs in the past 30 days in both the 2021 and 2022 surveys. First, participants who smoked CCs exclusively for more than 30 days per month in the 2021 survey were defined as “daily CC smokers.” Next, daily CC smokers in the 2021 survey were categorized into four groups based on their smoking status in the 2022 survey: continuous CC smokers (those who continued to smoke only CCs for 30 or more days per month), switching to dual users (HTP-dominant, those who used both CCs and HTPs for 30 or more days per month, with HTPs being used more days per month than CCs), switching to dual users (CC-dominant, those who use both CCs and HTPs for 30 or more days per month, with CCs being used more days per month than HTPs), and quitter/reduced smokers (those who completely quit smoking CCs or reduced their smoking frequency to fewer than 30 days per month).

Based on previous studies and clinical knowledge (14), the following variables were included as covariates: sex (male/female), age, educational attainment (junior high school/high school, vocational/technical college/junior college, or university/graduate school), occupational status (employer or unemployed), marital status (married, never married, or widowed/divorced), equivalent income (<14,000 USD, 14,000–28,000 USD, ≥28,000 USD, or declined to answer), alcohol consumption (never, former, or current drinker), diabetes mellitus status (no or yes), periodontal pain (no or yes), and smoking frequency (number of days per month the participants smoked CCs in the 2021 survey). For smoking frequency, we included the number of days the participants smoked CCs per month measured in the pre-baseline survey (2021) as a confounding variable. Smoking intensity measured at the baseline survey (2022) was not included because it was assessed concurrently with exposure and therefore did not precede it; thus, baseline smoking intensity could not be considered a prior confounding factor (Figure 1). Periodontal pain has been previously reported as a proxy indicator of periodontal disease severity (22). Therefore, periodontal pain from the 2021 survey was included to better account for disease severity at pre-baseline.

We employed the modified Poisson regression model with robust sandwich standard error to estimate risk ratios (RRs) and 95% confidence intervals (CIs) (23), evaluating the association between changes in smoking status from 2021 to 2022 and the transition from periodontitis in 2021 to severe periodontitis in 2023. We constructed both crude and adjusted models, with all covariates included in the adjusted analyses. Statistical significance was set at alpha = 0.05.

To address potential selection bias, we conducted a sensitivity analysis using inverse probability weighting (IPW) (24). Propensity scores were estimated using the covariates included in the primary analyses. Weighted modified Poisson regression models were then fitted to evaluate the robustness of the findings.

A directed acyclic graph (DAG) in Figure 1 was used to visualize the assumed causal structure among the exposure, confounders, and outcome. All statistical analyses were performed using Stata/MP version 18.0 (Stata Corp., College Station, TX, USA).

This prospective cohort study demonstrated that switching from daily CC smoking to dual use of CCs and HTPs was associated with an increased risk of severe periodontitis. However, no significant association was observed between transitioning from daily CC smoking to quitting or reducing smoking and the risk of severe periodontitis. Notably, the findings indicated that switching from daily CC smoking to dual use of CCs and HTPs may increase the risk of severe periodontitis rather than reduce it.

Our findings align with those of a previous cross-sectional study, which reported that dual users of CCs and HTPs had a higher prevalence of periodontal disease compared to CC-only smokers, using non-smokers as the reference group (14). The increased risk of periodontal disease in dual users of CCs and HTPs compared to CC-only users may be attributed to multiple factors. For individuals who switched from daily CC smoking to dual use of CCs and HTPs, their periodontal health is likely influenced by the amount of CCs they smoked in the baseline survey and the additional toxic effects of HTP use (15–18). Consequently, the dual use of CCs and HTPs may exacerbate the risk of severe periodontitis, making it worse than for CC-only smokers. Moreover, the difference in the risk of severe periodontitis between dual users of HTP-dominant and CC-dominant types in our study could be explained by differences in CC exposure. Dual users of CC-dominant type smoked CCs for an average of 30.6 days, approximately 10 days longer than the average 20.0 days for dual users of HTP-dominant type in the baseline (2022) survey. This greater exposure to CC smoke is thought to be linked to the higher risk of severe periodontitis among dual users of CC-dominant type.

In contrast, our study found no significant difference in the risk of severe periodontitis between quitters/reduced smokers and continuous CC smokers. These findings partially differ from previous studies, which reported a lower prevalence of periodontal disease among individuals who had quit smoking for longer periods (25–27). A possible explanation for this discrepancy is the shorter duration of smoking cessation or reduction in our study, which was limited to 1 year, compared to the 10 years or more considered in earlier studies. Future research involving longer durations of smoking cessation or reduction may reveal a lower risk of severe periodontitis among quitters/reduced smokers compared to continuous CC smokers.

Our findings suggest that switching to dual use of CCs and HTPs can have harmful effects on periodontal tissues, similar to those caused by CC smoking. Previous cohort studies have demonstrated that switching to dual use may increase the likelihood of relapse into regular smoking (28) and is not effective in supporting smoking cessation (29). From a clinical dentistry perspective, this highlights the importance of complete smoking cessation, rather than merely switching, to preserve and improve periodontal health. In addition, earlier studies have shown that periodontal treatment alone may not be effective in improving periodontal health for smokers (30, 31). Therefore, a combined approach that includes both smoking cessation therapy and dental treatment by dental professionals is essential for the effective management of periodontal health.

Our study has some limitations. First, the use of self-reported questionnaires may have introduced information bias. The measurements for smoking status (exposure) and periodontitis (outcomes) were based on subjective responses, which might not accurately reflect the actual conditions and could lead to exposure or outcome misclassification. To mitigate this concern, we adopted previously validated definitions of periodontitis and severe periodontitis and additionally adjusted for periodontal pain to account for baseline disease severity. Future research incorporating clinical assessment for periodontal status and detailed smoking status data alongside self-reported questionnaires could strengthen these findings. Second, regarding confounding, although we accounted for various potential confounders, unmeasured factors, such as the number of years the participants smoked CCs before the survey and the duration of smoking, may have influenced the results. However, we have attempted to address this issue by adjusting for the number of days participants smoked CCs before the baseline. Third, we used the number of days participants smoked CCs and used HTPs as a measure of CCs and HTPs consumption. However, future studies should incorporate more detailed data, such as the exact number of CCs and HTPs sticks consumed, to better clarify the association. Fourth, regarding generalizability, this study was based on an internet survey, and the participants could not be representative of national data. However, the national survey reported that 69.7% of current smokers in men and 63.2% of current smokers in women smoked only CCs in Japan (21), which is similar to our study participants, continuous CC smokers (67.4%). Finally, the strict inclusion criteria resulted in a relatively small analytic sample (n = 193), which may generate selection bias. We addressed this point by presenting a DAG and conducting sensitivity analyses using IPW, both of which supported the robustness of our main findings.

Our study has some notable strengths. First, we developed a comprehensive smoking trajectory framework, focusing on daily CC smokers, defined as individuals who smoked CC every day prior to the baseline. These participants were categorized into distinct smoking patterns, including switching dual users of HTP/CC-dominant type, based on the reported frequency of CC and HTP use in the baseline survey. Second, to the best of our knowledge, this is the first longitudinal study to assess the association between switching from daily CC smoking to dual use of CCs and HTPs and the risk of severe periodontitis. This longitudinal design provides valuable new evidence on the severity of periodontitis in relation to smoking status while addressing the potential for reverse causation.