The strong comorbidity between psychosis and substance use is already identifiable in early psychosis, raising the question of the direction of the association between substance use and psychosis onset. It has long been considered that this association was explained by the self-medication hypothesis. This hypothesis has been recently challenged by several prospective studies carried out in population-based samples, showing a dose-response relationship between cannabis exposure and risk of psychosis. This association was independent from potential confounding factors such as exposure to other drugs and pre-existence of psychotic symptoms. As a large percentage of subjects from the general population is now exposed to this drug, even a small increase in the risk of adverse effects may have significant deleterious consequences for the health of the population. Hence, reducing exposure to cannabis may contribute to prevention of some incident cases of psychosis. Regarding prognosis, persistent substance misuse after the onset of psychosis has a deleterious impact on clinical outcome. Therapeutic programs for subjects with dual diagnosis should be implemented early in the course of psychosis to maximise their impact on the course of illness.