Psychological Medicine

Original Articles

Cannabis use and childhood trauma interact additively to increase the risk of psychotic symptoms in adolescence

M. Harleya1a2, I. Kellehera1, M. Clarkea1, F. Lyncha2, L. Arseneaulta3, D. Connora1, C. Fitzpatricka2 and M. Cannona1 c1

a1 Department of Psychiatry, Royal College of Surgeons in Ireland and Beaumont Hospital, Dublin, Ireland

a2 Department of Child and Adolescent Mental Health, Mater Misericordiae University Hospital, The Children's University Hospital Temple Street, and University College Dublin, Dublin, Ireland

a3 Medical Research Council Social, Genetic and Developmental Psychiatry Centre, Institute of Psychiatry, King's College London, UK


Background Adolescent cannabis use has been shown in many studies to increase the risk of later psychosis. Childhood trauma is associated with both substance misuse and risk for psychosis. In this study our aim was to investigate whether there is a significant interaction between cannabis use and childhood trauma in increasing the risk for experiencing psychotic symptoms during adolescence.

Method Psychiatric interviews using the Schedule for Affective Disorders and Schizophrenia for School-Age Children (K-SADS) semi-structured instrument were carried out with 211 adolescents aged between 12 and 15 years and their parents as part of a population-based study. The interview enquired about early traumatic events, cannabis use and psychiatric symptoms in adolescence.

Results In separate analyses both cannabis use and childhood trauma were significantly associated with risk of experiencing psychotic symptoms. However, the presence of both childhood trauma and early cannabis use significantly increased the risk for psychotic symptoms beyond the risk posed by either risk factor alone, indicating that there was a greater than additive interaction between childhood trauma and cannabis use.

Conclusion Our finding of a greater than additive interaction between childhood trauma and cannabis use may have implications for the identification of individuals at high risk of experiencing psychotic symptoms. For example, measures to actively discourage or intensively treat cannabis use in children and adolescents who have experienced abuse may help to prevent the development of psychosis in this vulnerable group. Our findings require replication in larger samples to confirm this interaction effect.

(Received March 19 2009)

(Revised October 13 2009)

(Accepted October 13 2009)

(Online publication December 09 2009)