introduction Tobacco represents a major health risk for everyone, with a direct link between the risk of developing diseases and the duration of consumption. Young people are therefore key targets for tobacco prevention; the few searches carried out in this age group have not yet been able to identify effective means of preventing the start of cigarettes or stopping pre-existing consumption. The aims of this study are to identify the effects of short motivational intervention and to define the need for a booster in this particular category of the population, young men aged 19. Methods This study was conducted from October 2008 to September 2009 at the Lausanne Recruitment Centre, Vaud. Individuals were invited to participate in a short motivational intervention targeting simultaneously tobacco, alcohol and cannabis, without prior screening. The 823 volunteers were randomised between case and witness groups, with a new randomisation in the intervention group assigning a telephone booster three months later to certain cases. This work focuses only on the results for tobacco six months after the intervention. The effects (smoking and daily smoking status, average number of smoked cigarettes, dependence on cigarettes according to Fagerström criteria) were analysed on the basis of a written questionnaire at the baseline, followed by telephone contact at the six-month follow-up; they were analysed separately for different groups: all participants, smokers and daily smokers. Results Short motivational interventions have no significant effect on tobacco consumption. However, they have a positive trend in being a daily smoker and smoker, the average number of cigarettes consumed and the addiction according to Fagerström among all participants, as well as in the daily smoking subgroups. The results of the boosters are not significant and lead to mixed effects depending on the analyses carried out and the groups observed. Discussion The brief motivational interventions had no significant effect but seemed to be in favour of a reduction in tobacco consumption. Boosters do not seem to have a beneficial impact, but these conclusions should be weighed against the fact that none of the results obtained is significant. More systematic research is needed to better understand these disappointing results: with regard to interventions, the causes to be considered are the use of volunteers without prior screening, the targeting of intervention on multiple substances for a relatively short period of time, or the difficulty of targeting young people to the risks associated with cigarettes; as for boosters, an increase in frequency or duration could be an alternative, in order to make this complementary session effective.