A study published this month in the Archives of General Psychiatry concludes that "early cannabis use is associated with psychosis-related outcomes in young adults".
Compared with those who had never used cannabis, young adults who had 6 or more years since first use of cannabis (ie, who commenced use when around 15 years or younger) were twice as likely to develop a nonaffective psychosis and were 4 times as likely to have high scores on the PDI.
(Note: PDI refers to the Peters et al Delusions Inventory)
This study used sibling pairs to reduce the likelihood that unmeasured genetic and environmental factors explained the findings, a criticism that had been made of previous studies which also found a link between marijuana use and psuychosis.The seven previous studies finding this effect and referenced by the study are:
1. Andréasson S, Allebeck P, Engstrom A, Rydberg U. Cannabis and schizophrenia: a longitudinal study of Swedish conscripts. Lancet. 1987;2(8574):1483-1486.
2. Zammit S, Allebeck P, Andreasson S, Lundberg I, Lewis G. Self reported cannabis use as a risk factor for schizophrenia in Swedish conscripts of 1969: historical cohort study. BMJ. 2002;325(7374):1199.
3. van Os J, Bak M, Hanssen M, Bijl RV, de Graaf R, Verdoux H. Cannabis use and psychosis: a longitudinal population-based study. Am J Epidemiol. 2002;156(4):319-327.
4. Henquet C, Krabbendam L, Spauwen J, Kaplan C, Lieb R, Wittchen HU, van Os J. Prospective cohort study of cannabis use, predisposition for psychosis, and psychotic symptoms in young people. BMJ. 2005;330(7481):11.
5. Weiser M, Reichenberg A, Rabinowitz J, Kaplan Z, Caspi A, Yasvizky R, Mark M, Knobler HY, Nahon D, Davidson M. Self-reported drug abuse in male adolescents with behavioral disturbances, and follow-up for future schizophrenia. Biol Psychiatry. 2003;54(6):655-660.
6. Arseneault L, Cannon M, Poulton R, Murray R, Caspi A, Moffitt TE. Cannabis use in adolescence and risk for adult psychosis: longitudinal prospective study. BMJ. 2002;325(7374):1212-1213.
7. Fergusson DM, Horwood LJ, Swain-Campbell NR. Cannabis dependence and psychotic symptoms in young people. Psychol Med. 2003;33(1):15-21.
The popular culture seems to have embraced the idea that making an association between marijuana use and psychosis is ludicrous, as exemplified by the ridiculing of the 1930's film Reefer Madness. But now it seems contemporary medical studies confirm that film's eponymous claim.