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

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.