Introduction:
Public-health actors were alarmed by the progression of experimentation of e-cigarettes in different countries since 2012 because of fear that they would be a gateway to tobacco use. In Europe, there is now near a consensus that ecigarettes need supervision as required by the TPD (control of products and advertising) or not (sales to minors). But the question "is the ecigarette a gateway to smoking or, conversely, a competitor to smoking among young people?" has not yet received a definitive scientific answer.

Material and Methods:
A prospective cohort is not adequate to answer the question, because the susceptibility to the drug is preexistent to first use and cannot be predicted. In cross sectional studies,if the e-cigarette is a gateway to smoking among teenagers: - tobacco consumption should increase with e-cigarette experimentation, - teenagers should massively use nicotine e-cigarette, - conversion from experimentation to regular use will be high, - the e-cigarette use among smokers should be low. If e-cigarette is a competitor of tobacco one expects: - decrease in tobacco consumption, - use of non-nicotine products, - low switch rate from experimentation to regular use, - high rate of use among ex-smokers.

Results:
Whether on USA, UK or Paris data’s (2017 results will be presented), we observe: - smoking rate decrease. - nicotine-free e-liquid use, - low rate of regular use among experimenters, - high rate use in smokers and ex-smokers.

Conclusions:
Data’s are compatible with a competitive effect and no compatible with a gateway effect.