Abstract : In spite of its soft drug reputation, severe cannabis abuse can produce a number of adverse chronic effects. Whereas the majority of consumers make a « soft » use of cannabis, there is a minority of problematic cannabis users. However, many of cannabis chronic effects are still controversial, especially regarding the causal nature of their relationship with cannabis use. There is a scientific consensus to claim that cannabis induces a state of dependence in a small proportion of users. Severe abuse of cannabis can also lead to cognitive impairments, especially on memory, although these effects usually improve after the cessation of cannabis use. The statistical link between cannabis use and the development of psychotic disorders is more worrying, although the causal nature of this relationship remains controversial. Finally, a chronic abuse of cannabis is reputed to induce an amotivational syndrome, mainly characterized by a state of apathy. Although the symptoms of the amotivational syndrome are in keeping with some clinical observations, it remains difficult to ascertain whether this clinical picture is causally produced by cannabis abuse.