There is increasing evidence that cannabis use and psychosis are linked with people who use cannabis in adolescence or young adulthood being more likely to suffer from psychotic symptoms later in life. People with psychosis are also more likely to suffer from other mental-health problems the most common of which is depression. An Australian study of 101 people with psychosis looked at the effect cannabis use had on their psychotic symptoms and their levels of depression. The researchers found that cannabis caused a small but statistically significant increase in symptoms of psychosis but not depression. The relationship did not work the other way insofar as people who were suffering from psychotic symptoms or depression were not more likely to use cannabis. This goes against the self-medication hypothesis which argues that people use cannabis to relieve their mental-health problems.
Degenhardt, Louisa ... [et al] - The temporal dynamics of relationships between cannabis, psychosis and depression among young adults with psychotic disorders : findings from a 10-month prospective study Psychological Medicine July 2007, 37(7), 927-934