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. Keypoints In the course of adolescence, behaviors in the consumption of cigarettes or cannabis connect with the relational context in which young people evolve at school community level The product-based approach (tobacco or cannabis) is less effective than an approach through analysis of both the conflicts and the nature of peer group meet-ups Analysis of parent-adolescent conflicts and control of meet-ups seem to be powerful levers of action for prevention of embarking on the use of products The Ageven grid for the description of life events is an innovative tool used to analyses a situation within the community as well as for the development of primary prevention