People who smoke high-potency marijuana, especially if they do so daily, are at a three to five times greater risk of experiencing hospitalization for psychosis than people who smoke low-potency marijuana or no marijuana at all, according to research in The Lancet Psychiatry.
The team of Kings College London researchers studied 410 patients with first-episode psychosis and 370 population controls in an area of South London, England between May 1, 2005, and May 31, 2011.
Researcher Sir Robin Murray told The Guardian that the study provided “clear evidence” for a “causal link” between smoking strong cannabis and the risk of mental illness. “The argument initially was that the people who are going to smoke cannabis are a bit odd anyway,” said Murray. “In south London, two-thirds of people have used cannabis and it seems unlikely that two-thirds of people are abnormal.”
Smoking skunk cannabis triples risk of serious psychotic episode, says research (The Guardian, February 16, 2015)
Di Forti, Marta, Arianna Marconi, Elena Carra, Sara Fraietta, Antonella Trotta, Matteo Bonomo, Francesca Bianconi, et al. “Proportion of Patients in South London with First-Episode Psychosis Attributable to Use of High Potency Cannabis: A Case-Control Study.” The Lancet Psychiatry. Accessed February 17, 2015. doi:10.1016/S2215-0366(14)00117-5. (Abstract and full text)