There has been a lot of debate about the links between cannabis and psychosis and in particular the effects of high-strength 'skunk' cannabis which contains much more of the active ingredient delta-9-tetrahydrocannabinol. Researchers from the Institute of Psychiatry, King's College London compared 280 people turning up for treatment with their first episode of psychosis with 174 healthy people recruited from the local population. There was no difference between the two groups in terms of whether they had ever taken cannabis but people with psychosis were over six times more likely to be current daily users of cannabis and over twice as likely to have smoked cannabis for more than five years. 78% of the people with psychosis smoked skunk compared to only 37% of the control group.
Di Forti, Marta ... [et al] - High-potency cannabis and the risk of psychosis British Journal of Psychiatry December 2009, 195(6), 488-491