The availability of high potency (skunk-like) cannabis in south London has been linked to a greater proportion of people developing first onset psychosis, reports a new study published online in The Lancet Psychiatry.  In the UK, the incidence of schizophrenia has doubled in south London since 1965 and researchers believe that part of this increase may have to do with the increase in cannabis use among those who developed schizophrenia.

The authors reviewed data from 410 patients who developed first-episode psychosis and compared it to 370 population controls. Compared with subjects who never used cannabis, users of skunk-like cannabis were almost three times more likely to have a psychotic disorder (adjusted odds ratio [OR]: 2.92.  For those who used skunk-like cannabis on a daily basis, the odds of having a psychotic disorder were even greater (adjusted OR: 5.4).

Investigation of samples seized in London showed that skunk contained very high concentrations of THC and almost no cannabidiol compared to hash.  High concentrations of THC may impart greater detrimental effects on mental health while cannabidiol has been shown to reduce the psychotogenic effect of THC, and may even have antipsychotic properties. This may be the reason why hash users did not show an increase in psychotic disorders compared to non-users.

Given the worldwide movement towards relaxing the legal restraints of marijuana use, and with more varieties of cannabis becoming available, the authors conclude that raising public awareness of the risks associated with high-potency cannabis is critical.