Marijuana study: Does smoking skunk really cause psychosis?

New research found people smoking hash were less likely to have psychotic episodes

A Seattle resident takes marijuana from a plastic bag (Stephen Brashear/Getty Images)

There’s a new study on the relationship between cannabis and psychosis, the apparent conclusion of which is that super-strong skunk is causing around one in four new psychosis cases in the UK.

Data obtained from 780 south Londoners, more than half of whom were patients with first-episode psychosis, suggests that people who smoke particularly potent marijuana are more likely to experience a psychotic episode than those who don’t — or those who smoke hash instead.

It is, however, problematic to extrapolate on this small-sized sample, chosen specifically because of the area’s notorious cannabis predilection; the researchers themselves state their conclusions carefully.

As ever with these sorts of studies, there remain questions over the reliability of self-reporting and whether these skunk smokers were already psychologically susceptible.

Though it might be a step to far to use as evidence that weed is to blame for a quarter of ‘all new serious mental disorders’ – as the Mail on Sunday did – the findings are significant for the study's interrogation of pot potency and use frequency.

This is reportedly down to the high concentration of THC, coupled with the near-absence of potentially antipsychotic cannabidiol more frequently found in hash.

Fewer hash smokers participated in the study, but it is striking that those who did – regularly or irregularly – were more likely to not be psychotic than the other way around.

On the other hand, skunk smokers are seemingly far more at risk – with daily users five times more likely to experience first-episode psychosis.

It’s hard to say how many of those are smoking skunk, though anecdotal evidence suggests it’s the main substance sold in London.

Dr Di Forti told BBC Radio 4: "In London, it's very difficult to find anything else."