Experimenting first with e-cigarettes versus first with cigarettes and transition to daily cigarette use among adolescents: the crucial effect of age at first experiment.

Background And Aims: Most studies in English-speaking countries have found a positive association between e-cigarette experimentation and subsequent daily tobacco smoking among adolescents. However, this result may not be valid in other cultural contexts; in addition, few studies have assessed whether this association varies with the subject' age at the time of e-cigarette experimentation. This study aimed to estimate the association between experimenting first with e-cigarette (rather than tobacco) and subsequent daily smoking according to age at the time of experimentation.

Design: Secondary analysis; risk ratios (RRs) computed using modified Poisson regressions with inverse probability weighting.

Setting: A cross-sectional nation-wide representative survey performed in 2017 in France.

Participants: French adolescents (n = 24 111), aged 17 to 18.5 years, who had previously experimented with either e-cigarettes or tobacco.

Measures: Exposure was defined as the experimentation with e-cigarettes first (whether or not followed by experimentation with tobacco); the outcome as daily tobacco smoking at the time of data collection. Gender, age, literacy, socio-economic status, pre-exposure repeat school years and experimentation with drunkeness, 3 licit and 8 illicit drugs were adjusted for. Uncertainties about the sequence of events defining exposure were handled by the definition of three patterns of exposure, to avoid a misclassification bias.

Findings: Exposure reduced the risk of transition to daily smoking: RR = 0.58, 95% confidence interval (CI) = 0.54, 0.62. This effect increased in a linear manner with age at exposure (RR = 0.87, 95% CI = 0.78; 0.98 for 1 year, P < 0.001): from RR = 1.30, 95% CI = 1.09; 1.54 at age 9 to RR = 0.38, 95% CI = 0.32; 0.45 at age 17.

Conclusions: Experimenting with e-cigarettes first (as opposed to tobacco first) appears to be associated with a reduction in the risk of daily tobacco smoking among French adolescents aged 17-18.5, but this risk varies negatively with age at experimentation, and early e-cigarette experimenters are at higher risk.