Daily use of cannabis is associated with a much higher risk of developing psychosis, according to a case-control study at the Lancet Psychiatry.
some 900 adults who presented the first episodes of psychosis in 11 places in Europe and Brazil were compared with 1200 controls based on a population without psychosis.
Participants completed questionnaires on the use of cannabis throughout their lives; in the analyzes the reported tetrahydrocannabinol (THC) concentration in cannabis in different countries was also taken into account.
In general, daily cannabis use was three times more likely to be psychosis compared to never used or sporadically used. The daily use of high-powered marijuana (10% or more THC) was associated with a nearly five-fold increase in risk.
The researchers write: "Assuming a causal relationship, the percentage of new cases of psychotic disorders in the total sample attributed to daily use was 20.4%." One of the commentators is not convinced of causality, noting: "It is entirely possible that the relationship between marijuana and psychosis is bi-directional, as other genetic variables suggest."
The use of cannabis is associated with an increased risk of subsequent psychotic disorders, but it is not yet clear whether this affects the incidence of the disorder.
Our goal was to identify patterns of cannabis use with the greatest impact on the probability of psychotic disorders in Europe and to investigate whether differences in such patterns contribute to differences in the prevalence of psychotic disorders.
Patients from 18 to 64 years were included in the study psychiatric services in 11 places in Europe and Brazil with the first episode of psychosis and recruited controls representative of local populations.
We used logistic regression models adapted to the data to estimate which patterns of cannabis use were most likely to occur in psychotic disorders.
Using national and European data on the expected concentration of ?9-tetrahydrocannabinol (THC) in the different types of hemp available in the plant, we divided the types of cannabis used by the participants into two categories: low potency (THC Research Foundation of São Paulo, National Institute for Health Research (NIHR), Center for Biomedical Research (BRC) in South London and Maudsley Foundation NHS Trust and King s College London and NIHR BRC at University College London, Wellcome Trust.