Background: Friends play an important role in cigarette and cannabis usage during adolescence. However, family is also a factor insofar as it can expose an adolescent to the development of consumption, or protect them from it. Experimentation with, and consumption of, these substances comes about within a specific relationship configuration: that of a somewhat conflictual relationship between young people and their parents. Methods: The study was conducted in the Paris area (France) among 15-18-year-olds enrolled at school, with whom we conducted 93 biographical interviews (representing 483 person-years of retrospective observation). Results: The consumption of cigarettes or cannabis is influenced by relationships with peers and meet-ups with friends (especially at weekends), as well as within the protective school environment. The relationship between adolescent consumption and parental attitude is two-way; consumption can be considered as much a cause of conflict as it is a consequence. Conclusions: First, a product-based approach (tobacco or cannabis) is less fruitful than an approach of entry to addiction by life events. Secondly, analysis of both parent-adolescent conflicts and outings with friends seem to be powerful levers of action in preventing take-up of cigarettes and cannabis.