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

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

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.

More than half of the psychosis patients surveyed said they smoked skunk to some extent (53%) whilst skunk smokers only represented 29% of the non-psychotic control group.

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.

According to these stats, hash-smokers are even less at risk of first-episode psychosis than those who say they've never used cannabis.

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

Official government stats say that cannabis use by 18-59 year olds has stood steady at around 30% for the last 15 years.

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."