I've been a skeptic about the potential role of cannabis as an independent risk factor for developing psychosis, but this new study addresses some of the weaknesses of previous studies and shows a strong association between persistent cannabis use and the later development of psychosis. They only studied cannabis use that started after the study baseline to eliminate the effects of preexisting use, and they excluded anyone at baseline with any psychotic symptoms. The authors suggest that the risk might involve increasing the persistence of subclinical psychotic phenomena that would otherwise be transient, an interesting hypothesis. Underlying that suggestion is recent evidence that psychotic phenomena exist along a continuum from minor transient symptoms (quite common) to persistent and then more severe symptoms. Also note that the risk associated with cannabis was independent of family history of psychosis. There are still many unanswered questions about this link, but the evidence for it is growing and is strong enough now that warning parents and adolescents is probably warranted. Here's the link to the article.
BMJ 2011; 342:d738
Continued cannabis use and risk of incidence and persistence of psychotic symptoms: 10 year follow-up cohort study
Rebecca Kuepper, research psychologist1, Jim van Os, professor1, visiting professor2, Roselind Lieb, professor34, Hans-Ulrich Wittchen, professor45, Michael Höfler, research statistician5, Cécile Henquet, lecturer1
+ Author Affiliations
1Department of Psychiatry and Neuropsychology, South Limburg Mental Health Research and Teaching Network, EURON, Maastricht University Medical Center, Maastricht, Netherlands
2King’s College London, King’s Health Partners, Department of Psychosis Studies, Institute of Psychiatry, London, UK
3Department of Psychology, Division of Epidemiology and Health Psychology, University of Basel, Switzerland
4Max Planck Institute of Psychiatry, Munich, Germany
5Institute of Clinical Psychology and Psychotherapy, Technical University Dresden, Germany
Correspondence to: J van Os, Department of Psychiatry and Neuropsychology, South Limburg Mental Health Research and Teaching Network, EURON, Maastricht University Medical Center, PO Box 616, NL-6200 MD, Maastricht, Netherlands firstname.lastname@example.org
Accepted 31 December 2010
Objective To determine whether use of cannabis in adolescence increases the risk for psychotic outcomes by affecting the incidence and persistence of subclinical expression of psychosis in the general population (that is, expression of psychosis below the level required for a clinical diagnosis).
Design Analysis of data from a prospective population based cohort study in Germany (early developmental stages of psychopathology study).
Setting Population based cohort study in Germany.
Participants 1923 individuals from the general population, aged 14-24 at baseline.
Main outcome measure Incidence and persistence of subthreshold psychotic symptoms after use of cannabis in adolescence. Cannabis use and psychotic symptoms were assessed at three time points (baseline, T2 (3.5 years), T3 (8.4 years)) over a 10 year follow-up period with the Munich version of the composite international diagnostic interview (M-CIDI).
Results In individuals who had no reported lifetime psychotic symptoms and no reported lifetime cannabis use at baseline, incident cannabis use over the period from baseline to T2 increased the risk of later incident psychotic symptoms over the period from T2 to T3 (adjusted odds ratio 1.9, 95% confidence interval 1.1 to 3.1; P=0.021). Furthermore, continued use of cannabis increased the risk of persistent psychotic symptoms over the period from T2 to T3 (2.2, 1.2 to 4.2; P=0.016). The incidence rate of psychotic symptoms over the period from baseline to T2 was 31% (152) in exposed individuals versus 20% (284) in non-exposed individuals; over the period from T2 to T3 these rates were 14% (108) and 8% (49), respectively.
Conclusion Cannabis use is a risk factor for the development of incident psychotic symptoms. Continued cannabis use might increase the risk for psychotic disorder by impacting on the persistence of symptoms