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Marta Di Forti, MD, MRCPsych, Clinical Lecturer at the Institute of Psychiatry at King's College London, et al., stated in their article titled "Proportion of Patients in South London with First-Episode Psychosis Attributable to Use of High Potency Cannabis: a Case-Control Study," published online by Lancet Psychiatry on Feb. 18, 2015:

“We aimed to investigate how frequent use of skunk-like (high-potency) cannabis in south London affected the association between cannabis and psychotic disorders…

Compared with those who never used cannabis, individuals who mostly used skunk-like cannabis were nearly twice as likely to be diagnosed with a psychotic disorder if they used it less than once per week… almost three times as likely if they used it at weekends… and more than five times as likely if they were daily users…

The association between cannabis use and increased risk of developing schizophrenia-like psychosis has been consistently reported by prospective epidemiological studies. Our previous study was the first to show that use of high-potency (skunk-like) cannabis carries the highest risk for psychotic disorders. In the present larger sample analysis, we replicated our previous report and showed that the highest probability to suffer a psychotic disorder is in those who are daily users of high potency cannabis. Indeed, skunk use appears to contribute to 24% of cases of first episode psychosis in south London.”

Feb. 18, 2015