A great deal of previous research has suggested a link between cannabis use and psychotic episodes. New research suggests that this association is due mostly to shared environmental factors rather than direct causation or shared genetic risk factors.

In the first direct test of whether the link between cannabis use and psychosis is due more to environmental or genetic factors, Sania Shakoor of the University of London and colleagues show that environmental factors can account for the co-occurrence of cannabis use and paranoia, cognitive disorganization, and negative symptoms (flat affect, low motivation). Their results are published in the June issue of the journal Psychiatry Research.

Previous work has shown a strong link between cannabis use (smoking marijuana) and the development of psychosis. Overall, a regular cannabis user has approximately double the risk of developing a psychotic disorder than a non-user. The cause of this connection, however, is unknown.

Most cannabis users do not develop psychosis and although the risk of developing psychosis following cannabis use seems to run in families, this could still be due to shared genetic effects or common environmental risk factors. It could be the case that cannabis use alters one’s brain chemistry (and that some families are more at risk than others), or that particular environmental factors like socioeconomic disadvantage or trauma lead independently to increased rates of cannabis use and psychosis.

This study surveyed 4,830 families of adolescent twins, including both monozygotic and dizygotic twins, of whom 9.4% stated that they had previously tried cannabis. As in previous studies, cannabis use was associated with more psychotic episodes, but overall accounted for little variation in psychotic events. Little difference between monozygotic and dizygotic twins suggested that environmental effects are more important than genetic effects in explaining the connection between cannabis use and psychosis.

“Cannabis use co-occurs with psychotic events in adolescence due to environmental risk factors that are common to both. These data argue against the hypothesis that psychotic experience and cannabis use co-occur due to a similar underlying genetic liability in adolescence and highlight adolescence as a developmental period where environmental processes are significant,” the authors concluded. Future work will have to pinpoint the particular environmental factors that are contributing to these conditions.