Cohort study of electronic cigarette use: safety and effectiveness after 4 years of follow-up.

More than a decade after e-cigarette (e-cig) market launch, limited information are available on their safety after 24 months of use. In 2013, we started the first observational study assessing e-cig long-term effectiveness and safety, directly comparing tobacco smokers and e-cig users. Here we report the results after four years of follow-up. Adults (30-75 years) were included if: smokers of ≥1 tobacco cigarette/day (tobacco smokers); users of any type of e-cig inhaling ≥50 puffs weekly (e-cig users); users of both tobacco and e-cig (dual users). Data were collected by phone and/or internet, and carbon monoxide levels tested in 50% of those declaring tobacco abstinence. Main outcomes were: possibly smoking-related diseases (PSRD; validated through hospital discharge data or visit in 62.6% of the sample); 4-year tobacco abstinence; number of tobacco cigarettes/day. Data were available for 228 e-cig users (all ex-smokers), 471 tobacco smokers, 216 dual users. A PSRD was observed in 73 subjects (8.0%). No differences emerged across groups in PSRD rates, with negligible variations in self-reported health. Of e-cig users, 63.6% remained tobacco abstinent; dual users and tobacco smokers showed non-significantly different rates of tobacco (33.8% vs. 26.8%) and all-product (20.2% vs. 19.4%) cessation, and a similar decrease in cigarettes/day. Almost 40% of the sample switched at least once (tobacco smokers: 17.2%; dual users: 81.9%). After four years, a scarce, non-significant harm reduction was observed among e-cig or dual users. Given the long-lasting health effects of tobacco smoking, the benefits of e-cig use may start being detectable at the next follow-up (six years). The complete switch to e-cig may help tobacco quitters remain abstinent, but e-cig use in addition to tobacco did not increase the likelihood of smoking cessation or reduction.