Cannabis use in first episode psychosis: Meta-analysis of prevalence, and the time course of initiation and continued use.
OBJECTIVE
Cannabis use is prevalent among people with first episode psychosis and the epidemiology of its use in early psychosis is unclear. We performed a meta-analysis of observational studies to determine; (1) the interval between age at initiation of cannabis use and age at onset of first episode psychosis, (2) the prevalence of cannabis use at time of first episode psychosis, and (3) the odds of continuing cannabis following treatment for first episode psychosis.


METHODS
Search of electronic databases MEDLINE, EMBASE, PsycINFO, Web of Science and CINAHL for English-language papers using search terms (psychosis OR schizophrenia) AND (cannabis OR marijuana) IN (title OR keyword OR abstract), current to October 2014.


METHODS
Studies were included if they reported on prevalence of current cannabis use in first episode psychosis cohorts. A total of 37 samples were included for meta-analysis.


METHODS
Rates of cannabis use in each sample were extracted to determine prevalence estimates. The age at initiation of regular cannabis and age at onset of psychosis were used to determine the length of cannabis use preceding psychosis. Prevalence estimates at first episode psychosis and various time points of follow-up following first episode psychosis were analysed to determine odds ratio of continuing cannabis use. Data synthesis was performed using random-effects meta-analyses.


RESULTS
The pooled estimate for the interval between initiation of regular cannabis use and age at onset of psychosis was 6.3 years (10 samples, standardised mean difference = 1.56, 95% confidence interval = [1.40, 1.72]). The estimated prevalence of cannabis use at first episode psychosis was 33.7% (35 samples, 95% confidence interval = [31%, 39%]). Odds of continued cannabis use between 6 months and 10 years following first episode psychosis was 0.56 (19 samples, 95% confidence interval = [0.40, 0.79]).
School of Psychiatry, Adelaide University, Adelaide, SA, Australia Northern Adelaide Local Health Network, Adelaide, SA, Australia.
Hannah
H
Myles
The Queen Elizabeth Hospital, Woodville South, SA, Australia.
Nicholas
N
Myles
School of Psychiatry, University of New South Wales, Randwick, NSW, Australia The Euroa Centre, Prince of Wales Hospital, Randwick, NSW, Australia mmbl@bigpond.com.
Matthew
M
Large
D017668:Age of Onset; D006801:Humans; D008385:Marijuana Smoking / Q000009:adverse effects*; D011618:Psychotic Disorders / Q000453:epidemiology*
Cannabis; epidemiology; first episode; meta-analysis; psychosis