Impact of Cannabis and Other Drugs on Age at Onset of Psychosis
J Clin Psychiatry 2008;69(7):1210-1216
© Copyright 2015 Physicians Postgraduate Press, Inc.
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Objective: The aim of this study was to
investigate the relationship between age and cannabis use in patients with a first psychotic
episode, and to analyze the mediating effect of
comorbid use of other drugs and sex on age at onset of
Method: All consenting patients (aged 15
to 65 years) with a first psychotic episode
needing inpatient psychiatric treatment during a
2-year period between February 1997 and January
1999 were considered, confirming a total of 131 patients. Subjects were interviewed using the
Structured Clinical Interview for DSM-IV Axis I
Disorders, and clinical and demographic data were collected. We used general linear models with
age at onset as the response variable and survival
Cox models to confirm the results. Both a
multivariate linear model and the corresponding Cox
model were fitted with a covariate that summarizes
the most significant contributors that seemed to
decrease age at onset.
Results: Regarding the effect of cannabis
use, a significant gradual reduction on age at
onset was found as dependence on cannabis
increased, consisting in a decrement of 7, 8.5, and 12
years for users, abusers, and dependents,
respectively, with respect to nonusers (p = .004, p < .001,
and p < .001, respectively). Multivariate
analysis showed a clear effect of cannabis use on age
at onset, which was not explained by the use of other drugs or by gender. The finding was
similar in the youngest patients, suggesting that this
effect was not due to chance.
Conclusion: The major contribution of
this investigation is the independent and strong
link between cannabis use and early age at onset
of psychosis, and the slight or nonexistent effect
of sex and comorbid substance abuse in this variable. These results point to cannabis as a
dangerous drug in young people at risk of