Prevalence of cigarette smoking and snus use
Tobacco use information was obtained on 31 213 participants, slightly more than half of whom were female (52.5%). The mean ages at interview (± standard deviation) for each gender were similar: 53.7 (± 5.8) years for males and 53.8 (± 5.8) years for females. presents the prevalence of cigarette use and snus use separately for the total population and by gender. Of the total study population, 63% of participants reported ever smoking cigarettes in their life-time. At the time of interview 39.3% were former smokers, while 23.7% were current smokers. Approximately half the participants described themselves as ‘regular’ ever smokers, while 11.9% reported smoking ‘now and then’ or ‘at parties’.
Distributions of cigarette smoking and snus use in the Swedish Twin Registry.
The life-time prevalence of cigarette smoking differed significantly by gender. More males reported ever smoking than females. However, more males than females had quit smoking at the time of interview (42.8% versus 36.1%). The age-adjusted POR for former versus never smoking comparing males to females was 1.33 (95% CI: 1.27–1.39), while the age-adjusted POR for current versus never smoking comparing males to females was 0.80 (95% CI: 0.76–0.85). Regular smoking was more common among males, while similar proportions of males and females smoked ‘now and then’ and at parties.
The life-time prevalence of snus use in the total study population was 15.8%. At the time of interview, 5.9% of participants were former snus users, while 9.9% were current snus users. The prevalence of snus use differed dramatically by gender, with far more males than females reporting ever using snus (30.4% versus 2.5%), and far more males than females were current snus users (19.1% versus 1.5%). The age-adjusted POR for ever snus use comparing males to females was 18.0 (16.2–20.0). The age-adjusted POR for current snus use comparing males to females was 15.9 (95% CI: 13.9–18.2). Males were also more likely to report ‘regular’ snus use than females (POR: 22.4 (95% CI: 19.8–254). Given the low proportion of women who used snus in this population, we restricted further analyses involving snus use to males.
Ages at initiation of tobacco use
As shown in , age at onset of cigarette smoking occurred almost entirely before age 25 in this population, with an accelerated rate between 12 and 20 years of age. The median age at onset of cigarette smoking was somewhat earlier for males (16.0 years) than females (17.0 years) which is reflected in the survival curves; the curve for males is slightly above that of females. The log-rank test, which examined differences between genders within the framework of Cox’s proportional hazards model, was highly significant at P < 0.0001. also presents the age at onset of snus use curve among males in the STR. The median age at onset of snus use among males was 20 years. Onset of snus use occurred over a longer time span than that of cigarettes.
Age at onset of tobacco use for males and females in the Swedish Twin Registry
Transitions between forms of tobacco use
is a graphical depiction of transitions between tobacco use over time among 14 424 males in the STR. There were missing data for 390 men, who were excluded from this figure. The first column represents the type of tobacco males reported using initially (no use, cigarettes first, cigarettes and snus at the same time, and snus first), followed by a ‘transition’ column that reflects the number of men who moved from one type of tobacco use to another. Finally, the last column shows the number of tobacco users at the time of interview. The arrows between the columns depict transitions from one type of tobacco use (row) to another.
Transitions between tobacco use among 14 424 males in the Swedish Twin Registry
Beginning at the upper left corner of , the first row of this column shows that a large proportion of men in this population reported they had never used either form of tobacco (4237/14 424 = 29.4%). As the diagram progresses to the right, the proportion of men in the sample who became non-tobacco users increased over time as a result of quitting tobacco use.
The first circle in the second row is a combination of exclusive cigarette users (71.5%) and men who began using cigarettes before they used snus (28.5%). At the time of interview, 60.2% of the men who were exclusive cigarette users had become former smokers, while 39.8% continued to smoke.
In contrasting rows two, three and four, it is evident that most men began smoking cigarettes first, rather than using snus first, or cigarettes and snus simultaneously. However, as evidenced by the larger circle in the intermediate column of row three, the proportion of men who reported a combination of cigarette and snus use increased over time. The arrows radiating from the circles indicate that the majority of men who used both cigarettes and snus either quit tobacco use altogether (30.6%) or are currently using only snus (47.7%). It was less common for men who used both cigarettes and snus to continue such practices (14.3%), and even less common for these men to quit snus and currently use only cigarettes (7.4%).
The last row of depicts men who began using snus first. Only 21.9% of these men took up cigarette smoking later in life. Among men who used only snus in their lives, 67.1% were doing so at the time of interview while 32.9% of exclusive snus users had quit using snus.