Abstract
Recent prospective findings have shown that cannabis use by young people could be a risk factor for psychotic symptoms in
adulthood, but the long-term impact of cannabis abuse on the clinical features of declared schizophrenia remains to be explored.
We assessed the independent influence of cannabis abuse on the clinical symptoms of schizophrenia, after controlling for
frequently co-occurring addictive disorders. Patients with schizophrenia, and with (N= 66), or without (N= 139) cannabis abuse,
were compared for lifetime positive and negative symptoms, taking into account presence of any other addictive disorders. The
incidence of the abuse of drugs other than cannabis was nearly five times greater amongst patients with both schizophrenia and
cannabis abuse. When the analyses were limited to subjects with no other abuse, less avolution and fewer apathy symptoms were
still detected in patients with schizophrenia and cannabis abuse than in those with no abuse (p= 0.0001). In contrast, between-group
differences for positive symptoms were abolished when multiple substance abuses were taken into account.
The strong association between cannabis abuse and fewer negative symptoms in schizophrenia was thus replicated in this
sample, but once co-morbid addictive disorders had been controlled no influence of cannabis abuse on hallucinations was detected.
Distinguishing the effects of co-occurring addictive disorder(s) in patients with schizophrenia and cannabis dependence may thus
be important when attempting to analyse the impact of cannabis abuse.
© 2006 Elsevier B.V. All rights reserved.