Neurology Central

Daily cannabis use linked to increased risk of developing psychotic disorder

Researchers based at King’s College London (UK) have performed a Europe-wide study to analyze the effect of daily cannabis use on population rates of psychotic disorder. The study, which has been published in The Lancet Psychiatry, raises concerns over the legalization of cannabis.

Previous studies have determined that there is an association between cannabis use and psychosis. However, until now there have been no reports on whether cannabis use affects rates of psychosis at a populational level.

In order to estimate the incidence rate of psychosis, individuals with first episode psychosis were identified according to the records of mental health services at eleven sites across Europe. Information about identified patients’ history of cannabis use and other recreational drug use was then collected. The researchers then compared patients who had first episode psychosis with healthy controls to identify risk factors associated with psychosis.

Using data on delta-6-tetrahydrocannabinol (THC), the authors estimated the cannabis potency of the different types of cannabis the patients were recorded to have used. They classified the types as either high potency (above 10% THC) or low potency (below 10% THC).

The authors of the study then determined that individuals who used cannabis daily were three times more likely to have a diagnosis of first episode psychosis than those who had never used cannabis. When high potency cannabis was used, this increased to five times more likely to exhibit psychosis.

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In Amsterdam (Netherlands) and London (UK), high potency cannabis is widely available, this was reflected in the results of the study. For example, in Amsterdam, over 40% of cases with first episode psychosis were linked to daily cannabis use and over 50% of cases were linked to high potency cannabis use.

“Our findings are consistent with previous studies showing that the use of cannabis with a high concentration of THC has more harmful effects on mental health than the use of weaker forms. They also indicate for the first time how cannabis use affects the incidence of psychotic disorder at a population level,” commented Marta Di Forti (King’s College London).

The results of this study confirm previous reports that high-potency cannabis can have a detrimental effect on mental health.

“As the legal status of cannabis changes in many countries and states, and as we consider the medicinal properties of some types of cannabis, it is of vital public health importance that we also consider the potential adverse effects that are associated with daily cannabis use, especially high potency varieties,” concluded Di Forti.

Sources: Di Forti M, Quattrone D, Freeman TP et al. The contribution of cannabis use to variation in the incidence of psychotic disorder across Europe (EU-GEI): a multicentre case-control study. The Lancet Psychiatry. (2019) (Epub ahead of print);