a1 Department of Psychiatry, Academic Medical Centre, University of Amsterdam, Amsterdam, The Netherlands
a2 Amsterdam Institute for Addiction Research, Amsterdam, The Netherlands
a3 Department of Psychiatry, Erasmus University Medical Centre Rotterdam, The Netherlands
a4 Department of Psychiatry, Rudolf Magnus Institute of Neuroscience, University Medical Centre Utrecht, The Netherlands
a5 Maastricht University Medical Centre, South Limburg Mental Health Research and Teaching Network, Maastricht, The Netherlands
a6 Department of Psychiatry, University Medical Centre Groningen, University of Groningen, The Netherlands
Background Cannabis use is associated with an earlier age at onset of psychotic illness. The aim of the present study was to examine whether this association is confounded by gender or other substance use in a large cohort of patients with a non-affective psychotic disorder.
Method In 785 patients with a non-affective psychotic disorder, regression analysis was used to investigate the independent effects of gender, cannabis use and other drug use on age at onset of first psychosis.
Results Age at onset was 1.8 years earlier in cannabis users compared to non-users, controlling for gender and other possible confounders. Use of other drugs did not have an additional effect on age at onset when cannabis use was taken into account. In 63.5% of cannabis-using patients, age at most intense cannabis use preceded the age at onset of first psychosis. In males, the mean age at onset was 1.3 years lower than in females, controlling for cannabis use and other confounders.
Conclusions Cannabis use and gender are independently associated with an earlier onset of psychotic illness. Our findings also suggest that cannabis use may precipitate psychosis. More research is needed to clarify the neurobiological factors that make people vulnerable to this precipitating effect of cannabis.
(Received May 05 2011)
(Revised December 11 2011)
(Accepted January 04 2012)
(Online publication March 27 2012)
c1 Address for correspondence: Dr L. de Haan, Department of Psychiatry, Academic Medical Centre, University of Amsterdam, Meibergdreef 5, 1105 AZ Amsterdam, The Netherlands. (Email: firstname.lastname@example.org)