BackgroundRandomised controlled trials have shown some benefits to using e-cigarettes (ECs) to facilitate smoking reduction and cessation, but real-world observational studies have rarely confirmed this. The present study looked at EC use and smoking during a four-year longitudinal study of young men. It compares general population findings with a subgroup of individuals with a good prognosis for quitting smoking. MethodsThe smoking habits of 5353 young men at t1 (average 21.3 years old) were defined as either never-smoker, ex-smoker, initiates, relapsed smoker or persistent smoker. At follow-up (t2), smoking status was analysed using logistic regression, differences in the number of cigarettes smoked by persistent t1/t2 smokers were analysed using mixed linear models and the number of quit attempts was analysed using negative binomial models. ResultsAt the general population level, EC use had no beneficial effects on reducing or ceasing smoking. Non-smokers (never- and ex-) and smokers at t1 were more likely to be smokers at t2 if they had begun to use ECs (e.g. among persistent smokers OR=4.56, 95% CI [2.75, 7.58]), but not-significantly so if they had already used ECs at t1. Among smokers at t1, almost daily EC use at t1 was associated with a non-significant reduction in smoking at t2 (OR=0.74, 95% CI [0.33, 1.65]), but occasional EC use significantly increased smoking at t2 (OR=3.05, 95% CI [2.29, 4.06]). Both daily and occasional EC use increased smoking at t2 among t1 non-smokers. T2 smokers made more attempts to quit when using ECs at t2 (IRR=1.53, 95% CI [1.26, 1.85]). Beneficial effects were found among a subgroup of EC users with a good prognosis for quitting (using nicotine liquids and at least 2nd generation ECs, motivation to quit and daily EC use at t2, but not t1). ConclusionSome smokers may have benefitted from using ECs, but they were few. At the general population level, ECs are not predominantly used in a way, which might optimise reducing or ceasing smoking. Therefore, the public health effect on the general population of using ECs may be questionable, as may policy measures to facilitate EC use.