Cannabis, vulnerability, and the onset of schizophrenia: an epidemiological perspective

Objective: Second to alcohol, cannabis is the most frequently misused subst ance among patients with schizophrenia. The aim of this paper is to examine at early onset of psychosis whether the high comorbidity of schizophrenia and cannabis abuse is due to a causal relationship between the two disorder s. Previous studies have mostly included chronic patients or samples with m ixed stages of the psychotic illness. Method: In a German catchment area with a population of 1 500 000, a repres entative first-episode sample of 232 patients with schizophrenia was includ ed in the Age, Beginning and Course of Schizophrenia Study. By means of a s tructured interview, the Retrospective Assessment of the Onset of Schizophr enia, the onset and course of schizophrenic symptoms and of substance abuse was systematically assessed retrospectively. Information given by relative s validated the patients' reports. Results: Thirteen per cent of the sample had a history of cannabis abuse, w hich was twice the rate of matched normal controls. Male sex and early symp tom onset were major risk factors. While cannabis abuse almost always prece ded the first positive symptoms of schizophrenia, the comparison of the ons et of cannabis abuse and of the first (prodromal) symptoms of schizophrenia differentiated three approximately equal groups of patients: group 1 had b een abusing cannabis for several years before the first signs of schizophre nia emerged, group 2 experienced the onset of both disorders within the sam e month, and group 3 had started to abuse cannabis after the onset of sympt oms of schizophrenia. Conclusions: The vulnerability-stress-coping model of schizophrenia suggest s possible interpretations of these findings. Group 1 might suffer from the chronic deteriorating influence of cannabis reducing the vulnerability thr eshold and/or coping resources. Group 2 consists of individuals which are a lready vulnerable to schizophrenia. Cannabis misuse then is the (dopaminerg ic) stress factor precipitating the onset of psychosis. Group 3 uses cannab is for self-medication against (or for coping with) symptoms of schizophren ia, particularly negative and depressive symptoms. These patients probably learn to counterbalance a hypodopaminergic prefrontal state by the dopamine rgic effects of cannabis. The implications of these very preliminary result s include issues of treatment and prognosis, but replication studies are ne eded.