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High-potency cannabis increases the risk of psychosis
  1. Emma Barkus
  1. Correspondence to University of Wollongong, Wollongong, New South Wales, Australia; ebarkus{at}

Statistics from

ABSTRACT FROM: Di Forti M, Marconi A, Carra, E, et al. Proportion of patients in south London with first-episode psychosis attributable to use of high potency cannabis: a case-control study. Lancet Psychiatry 2015;2:233–38.

What is already known on this topic

Existing evidence suggests cannabis is a contributory (but not necessary) risk factor for psychosis.1 However, the increasing percentage of tetrahydrocannabinol (THC) in cannabis has further complicated this contentious debate. The risk attributable to cannabis with high levels of THC (high-potency cannabis) may require consideration separately from milder forms of cannabis. Indeed, high-potency cannabis has been cited as one possible cause for the incidence of schizophrenia doubling since 1965.2

Methods of the study

Di Forti and colleagues recruited 410 inpatients with a first episode of affective or non-affective psychosis (ICD 10) in South London between 2005 and 2011. A control sample of 370 healthy people was recruited in the same …

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  • Funding Brian and Behaviour Research Foundation, Young Investigator Award, GW Pharmaceuticals, In kind support.

  • Competing interests None declared.

  • Provenance and peer review Commissioned; internally peer reviewed.

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