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The single-centre retrospective observational study was conducted at the Centre for Treatment of Tobacco Dependence of the 3rd Medical Department at the General University Hospital in Prague, Czech Republic (Centre). The publication of results of our standard treatment of tobacco dependence was approved by the Ethics Committee of General University Hospital in Prague No. 30/13, 49/21, and 1254/22 IS, D. At baseline visit, all patients signed informed consent and agreed to the evaluation of their personal data in anonymised form for research purposes.

The study site

The Prague-based Centre provides an intensive specialized tobacco dependence treatment to, roughly, 400–500 patients each year and operates full time for smokers since 2005. Patients could have been referred to the treatment Centre by their physician or self-refer. The treatment is based on evidence-based guidelines and has typically been tailored to the individual patient’s needs [18,19,20]. The treatment is provided by a nurse and a physician both with completed professional medical training, and it consists of face-to-face counselling – psychobehavioural intervention and mostly recommended pharmacotherapy including nicotine replacement therapy, varenicline, cytisine, and/or bupropion. In frame of the personal history collection, patients were asked about mental disorders, and positive answers were sorted in the following categories: anxiety, depression, bipolar affective disorder, or schizophrenia. The diagnoses were not verified by psychiatrist. The initial psychobehavioural intervention lasted about 2 h and subsequent visits about 30 min, with 12-months follow-up as described elsewhere [21].

The treatment cohort and the data collection

The analysed dataset included 7,498 patients treated as outpatients from 2005 to 2021 during the 12-month follow-up period, with an average number of visits being 4.3 (SD 2.7). The participants had an average age of 44.0 years (SD 14.0 years), with 52% male and 48% female. All individuals were aged 18 or older, heavily dependent on smoking, with an average of 21.7 cigarettes/day (SD 11.3 cigarettes/day) and average score of 5.5 points (SD 2.4 points) in the Fagerström Test of Cigarette Dependence (FTCD) [22, 23].

For the sake of consistency, records from the years 2005, 2020 and 2021 were excluded. Additionally, individuals with missing key information during the overall period were excluded. Thus, for this analysis, we included only those with the date of the first visit falling before the start of COVID-19 epidemic (2006–2019), resulting in a sample size of N = 6,546.

The baseline visit lasted approximately 1 h and involved a basic medical examination along with data collection on demographics, smoking, smoking dependence characteristics, personal smoking and medical history, and self-reported psychiatric problems, current or past, such as anxiety, depression, bipolar affective disorder, schizophrenia, including the year of onset and psychiatric pharmacological treatment. The Beck Depression Inventory (BDI-II) was also administered during the baseline visit [24, 25].

The statistical analysis

The data was described by means of the descriptive statistics in the software R v. 4.1.1 for Windows. Age groups were created as below 26, 26–35, 36–45, 46–55, 56–65, and above 65. Trend in the prevalence of reported mental disorders was analysed using the multivariate Poisson regression analyses of the R v. 4.2.1 for Windows package.

The year of the initial visit of the smoker in the Centre was taken as either metric covariate or a factor to better illustrate the year-to-year trend. In the either case, point and interval estimates of rate ratios associated with potential covariates were calculated and plotted as based on the multivariate generalized linear model with log-link function and Poisson distribution [26]. To obtain counts (N) by each factor combination, the records were grouped by the aforementioned age groups, gender, and the year of the first visit.

The general multivariate Poisson regression model with age as a covariate and gender as additional analysed factor was constructed as follows:

$$\mathrm{log}{N}_{mental\_disorder}={\beta }_{0}+{\beta }_{1}Age+{\beta }_{2}{Gender}_{i}+{\beta }_{3}Year+\mathrm{log}{N}_{total}$$


where the i denotes the gender and the year is studied as a continuous covariate. Alternatively, to understand the effect of individual years, the model was also fitted as:

$$\mathrm{log}{N}_{mental\_disorder}={\beta }_{0}+{\beta }_{1}Age+{\beta }_{2}{Gender}_{i}+{\beta }_{3}{Year}_{j}+\mathrm{log}{N}_{total}$$


where the i denotes the gender and the j the respective year studied as a factor. Both regression models estimate the prevalence rate ratio (PRR), the associated 95% confidence interval (CI), and the p-value as presented in Table 2.