Does age of first cannabis use and frequency of use influence age of first-episode psychosis (FEP)?

Background: Cannabis is one of the most commonly used drugs among young people across Europe (EMCDDA data 2014), Moreover, it is one of the most abused illicit drugs among patients suffering from schizophrenia (Linszen et al., 1994) and, particularly, in patients at their first episode of psychosis (Donoghue et al., 2011). Furthermore, patients suffering from psychosis with a history of cannabis use have an earlier age of onset of psychosis (AOP) than those who never used it (Di Forti et al., 2013). We aim to investigate if the reported association between use of cannabis and AOP is consistent across to European samples with expected differences in pattern of cannabis use (i.e. age at first use, frequency of use) Methods: Cannabis is one of the most commonly used drugs among young people across Europe (EMCDDA data 2014), Moreover, it is one of the most abused illicit drugs among patients suffering from schizophrenia (Linszen et al., 1994) and, particularly, in patients at their first episode of psychosis (Donoghue et al., 2011). Furthermore, patients suffering from psychosis with a history of cannabis use have an earlier age of onset of psychosis (AOP) than those who never used it (Di Forti et al., 2013). We aim to investigate if the reported association between use of cannabis and AOP is consistent across to European samples with expected differences in pattern of cannabis use (i.e. age at first use, frequency of use) Results: In the total sample, N = 935, comparing FEP who were cannabis users with never users, we found a significant difference in mean AOP (cannabis users: 28.30 (9.05) vs. non-users: 34.94 (12.5), t = -9.32, Po0.001). Moreover, 58% of cannabis users started at age ≤16 years old, with mean age of onset of Psychotic Disorder (25.47, sd = 7.03), compared with those who started later (M = 25.47, sd = 10.05) (t = -9.42, Po0.001). When the sample was split in NE and SE groups, we found that NE sample the mean AOP in cannabis users was 28.12 (±8.42) and 34.18 (±12.68) non-users (t = -4.65, Po0.001). In SE sample the mean AOP in cannabis users is 29.02 (±9.62) and in never users is 35.55 (±11.61) (t = -5.75, Po0.001). All predictors are statistically significant (in NE sample age first use β = .31, t = 5.16, P = .000, frequency β = -1.80, t = -2.93, Po0.001; in SE sample age first use β = 0.41, t = 6.67, P = .000, frequency β = -2.87, t = -4.66, Po0.001). In SE, the percentage of variance explained in a regression model is 31% (R2adj = .30) vs 16% (R2adj = .15) of NE. Discussion: Our results support the association between cannabis use and younger AOP in both samples, but were not observed significant difference across Europe. Linear regression model on predictors (age of first use, frequency of use) analyzed in the NE and the SE clinical samples confirmed relationship of causality with dependent variable (AOP), with a higher percentage of explained variance in sample of SE than NE.