Association between age at onset of psychosis and age at onset of cannabis use in non-affective psychosis

Abstract

Introduction

Several studies have associated cannabis use with the development of schizophrenia. However, it has been difficult to disentangle the effects of cannabis from that of other illicit drugs, as previous studies have not evaluated pure cannabis users. To test whether the onset of cannabis use had an effect on the initiation of psychosis, we examined the time relationship between onset of use and onset of psychosis, restricting our analysis to a cohort of individuals who only used cannabis and no other street drugs.

Methods

Fifty‐seven subjects with non-affective psychoses who used cannabis prior to developing a psychosis were interviewed using the Diagnostic Interview for Genetic Studies (DIGS). The Family Interview for Genetic Studies (FIGS) was also used to interview a family informant about psychiatric illness in the patient and the entire family. Multiple linear regression techniques were used to estimate the association between variables.

Results

After adjusting for potential confounding factors such as sex, age, lifetime diagnosis of alcohol abuse or dependence, and family history of schizophrenia, the age at onset of cannabis was significantly associated with age at onset of psychosis (β = 0.4, 95% CI = 0.1–0.7, p = 0.004) and age at first hospitalization (β = 0.4, 95% CI = 0.1–0.8, p = 0.008). The mean time between beginning to use cannabis and onset of psychosis was 7.0 ± 4.3. Age at onset of alcohol use was not associated with age at onset of psychosis or age at first hospitalization.

Conclusion

Age at onset of cannabis is directly associated with age at onset of psychosis and age at first hospitalization. These associations remain significant after adjusting for potential confounding factors and are consistent with the hypothesis that cannabis could cause or precipitate the onset of psychosis after a prolonged period of time.

Introduction

A number of clinical studies have found that cannabis use is associated with an earlier age at onset of psychosis in polysubstance abusers (Compton et al., 2009, Dragt et al., 2010), but little attention has been paid to whether the onset of cannabis use is actually associated with the onset of a psychosis. This should be examined by studying subjects whose cannabis use preceded the initiation of psychosis and who have no history of any other drug use that could also lead to a psychosis. Unfortunately, most previous studies did not focus on people who solely abused cannabis (Barnes et al., 2006, Leeson et al., 2011). Thus, the objective of the current study was to focus solely on heavy cannabis users and study the association of age at onset of both cannabis use and psychosis as one measure of whether cannabis use is causally related to psychosis. If cannabis causes or precipitates the onset of psychosis, a significant association should be found between both ages of onset after adjusting for potential confounding factors.