The British Journal of Psychiatry
High-potency cannabis and the risk of psychosis
Marta Di Forti, Craig Morgan, Paola Dazzan, Carmine Pariante, Valeria Mondelli, Tiago Reis Marques, Rowena Handley, Sonija Luzi, Manuela Russo, Alessandra Paparelli, Alexander Butt, Simona A. Stilo, Ben Wiffen, John Powell, Robin M. Murray



People who use cannabis have an increased risk of psychosis, an effect attributed to the active ingredient Δ9-tetrahydrocannabinol (Δ9-THC). There has recently been concern over an increase in the concentration of Δ9-THC in the cannabis available in many countries.


To investigate whether people with a first episode of psychosis were particularly likely to use high-potency cannabis.


We collected information on cannabis use from 280 cases presenting with a first episode of psychosis to the South London & Maudsley National Health Service (NHS) Foundation Trust, and from 174 healthy controls recruited from the local population.


There was no significant difference between cases and controls in whether they had ever taken cannabis, or age at first use. However, those in the cases group were more likely to be current daily users (OR = 6.4) and to have smoked cannabis for more than 5 years (OR = 2.1). Among those who used cannabis, 78% of the cases group used high-potency cannabis (sinsemilla, ‘ skunk’) compared with 37% of the control group (OR 6.8).


The finding that people with a first episode of psychosis had smoked higher-potency cannabis, for longer and with greater frequency, than a healthy control group is consistent with the hypothesis that Δ9-THC is the active ingredient increasing risk of psychosis. This has important public health implications, given the increased availability and use of high-potency cannabis.

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