Participants who reported previous or past-30-day e-cigarette use at baseline reported a higher average number of e-cigarettes smoked in the past 30 days at follow-up (mean number of e-cigarettes smoked: never e-cigarette users = 1.86 [SE = 0.88]; previous users = 13.0 [SE = 6.34]; past-30-day users = 15.4 [SE = 4.39]) (Supplemental Fig 3A, Table 1). Participants reporting previous e-cigarette use but no use in the past 30 days at baseline smoked an average of 3.47 (95% CI: 2.46–4.91) times as many e-cigarettes at follow-up as baseline never e-cigarette users, and past-30-day e-cigarette users at baseline smoked an average of 5.42 (95% CI: 2.56–11.5) times as many e-cigarettes at follow-up in adjusted models.

Similar patterns were observed for the association of baseline cigarette use with subsequent smoking frequency, in which the most frequent smoking at follow-up was observed for baseline past-30-day cigarette smokers .

Elevated RRs were observed for use of e-e-cigarettes without nicotine (versus never e-cigarette users) and for use of e-e-cigarettes with 1 to 3 mg of nicotine (versus never e-cigarette users); estimates could not be calculated for higher levels of nicotine (because of small sample size and lack of model convergence) or for use of e-e-cigarettes for dripping (or not for dripping) versus never e-cigarette users.

In analyses restricted to participants reporting past-30-day use of e-e-cigarettes at baseline, adolescents using mods smoked an average of 5.11 (95% CI: 1.74–15.0) times as many e-cigarettes in the past 30 days at follow-up relative to adolescents using a penlike e-cigarette device at baseline (model 2, Table 2). Adolescents using low-nicotine–containing e-cigarette products smoked an average of 3.12 (95% CI: 0.82–11.9) times the number of e-cigarettes in the past 30 days at follow-up as those using e-e-cigarettes without nicotine, and those using high-nicotine–containing e-cigarette products smoked an average of 4.69 (95% CI: 1.12–19.6) times the number of e-cigarettes at follow-up (versus those not using nicotine). No association was observed for use of dripping and subsequent number of e-cigarettes smoked (RR = 1.07; 95% CI: 0.35–3.20).

Given the associations of both device type and nicotine with frequency of cigarette smoking, a third set of models included both predictors simultaneously. When modeling both device type and use of nicotine in one model, only the association of device type with number of e-cigarettes smoked at follow-up remained elevated and statistically significant.

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