Teenage cannabis exposure may not be associated with schizophrenia

A new study shows that cannabis exposure during adolescence may not be independently associated with either adult-onset psychosis or schizophrenia.

The study examined the relationship between adolescent cannabis use and adult-onset psychosis in a longitudinal co-twin control analysis. They identified no dose-response relationship in models that compared the one twin with greater cannabis use to the other using less in relation to psychosis-proneness in adulthood.

The researchers also reported that there were no potentially differing effects on the twin’s levels of cannabis exposure and risk of developing schizophrenia later in life.

Schizophrenia study

They wrote: “Epidemiological studies have repeatedly shown that individuals who use cannabis are more likely to develop psychotic disorders than individuals who do not. It has been suggested that these associations represent a causal effect of cannabis use on psychosis and that psychosis risk may be particularly elevated when use occurs in adolescence.

“This study, however, does not support these hypotheses, suggesting instead that observed associations are more likely due to confounding by common vulnerability factors.”

The researchers who are associated with the University of Minnesota Institute of Child Development concluded: “The results suggest this association is likely attributable to familial confounds rather than a causal effect of cannabis exposure. … Our results suggest that the threat of potential harm to adolescents via meaningful increases in the risk of long-term psychotic illness may be overstated.”

“Thus, clinical and public health interventions aimed at decreasing the prevalence and burden of psychotic illnesses may benefit from focusing their attention elsewhere.”

The research was published in the Journal of Abnormal Psychology.

How common is adult-onset schizophrenia?

The Royal College of Physicians (RCP) reported that the condition can affect around one in every 100 people over the course of their life. It affects men and women equally but rarely occurs before the age of 15. While it is not understood exactly what can cause the condition, there are a number of factors that may contribute to its development such as genes, stress and brain damage. The RCP also list cannabis misuse on their website as a potential cause.

Twins with schizophrenia

According to the RCP, if a twin has identical genetic make-up to their twin, including small pieces of DNA, then there is a 50:50 chance they will both develop schizophrenia.

Non-identical twins have different genetic make-up so the risk is lower but still slightly higher than other family member risk.

Read more: No evidence to suggest cannabis leads to decline in cognitive ability