ADDITIVE INTERACTION BETWEEN LIFETIME CANNABIS USE AND CHILDHOOD TRAUMA IN INCREASING THE RISK FOR PSYCHOSIS. A REPLICATION ANALYSIS ON A SAMPLE OF FIRST EPISODE OF PSYCHOSIS PATIENTS

Abstract

Background: Additive interaction between childhood trauma and cannabis use in increasing risk for psychotic disorders has been recently demonstrated in a prospective (Harley et al., 2009) and a cross sectional study (Konigs et al., 2011), although not replicated in another survey (Kuepper et al., 2011). This study aimed to analyse additive interaction between lifetime cannabis and severe child abuse in increasing the risk for psychosis in a sample of first episode psychosis patients and geographically matched controls. In addition, analyses were re-run separately for males and females in order to investigate whether the association was moderated by gender. Methods: The sample consisted of 231 patients with psychosis at their first admission to the South London and Maudsley Mental Health NHS Foundation Trust, and 214 controls. Severe childhood trauma was assessed using the Childhood Experience of Care and Abuse Questionnaire (Bifulco et al., 2005) and it was defined as history of any physical abuse involving injuries or any sexual abuse involving intercourse which occurred before age of 17 years. Lifetime cannabis use was assessed using the Cannabis Experience Questionnaire modified version (Di Forti et al., 2010). Results: Controlling for ethnicity, level of education, and psychiatric family history, both childhood abuse (Adj OR1.87, 95%CI 1.08-3.21) and lifetime cannabis use (Adj OR 1.89, 95%CI 1.18-3.00) were associated with an increased risk for psychosis. Moreover, the joint effect of early trauma and cannabis use (Adj OR 3.82, 95%CI 1.79-8.14) was higher than the effect of either variable alone, suggesting an additive interaction between them. Although cannabis × trauma interaction was observed both in males and females, the effect appeared stronger in the latter group. Discussion: Our findings support the previous literature about an additive interaction between childhood trauma and cannabis use in increasing risk for psychosis.