David M. Semple, MBBS, Consultant Psychiatrist at Hairmyres Hospital, Andrew M. McIntosh, MSc, MD, MPhil, MBChB, Clinical Reader in Psychiatry at the University of Edinburgh, and Stephen M. Lawrie, MD, MPhil, MBChB, Professor of Psychiatry and Neuroimaging at the University of Edinburgh, stated the following in their Mar. 2005 article "Cannabis as a Risk Factor for Psychosis: Systematic Review," published in the Journal of Psychopharmacology:

“Meta-analysis suggests that cannabis is a risk factor, increasing the chances of developing schizophrenia or a schizophrenia-like psychotic illness by approximately three-fold…

Our findings underline the need to recognize the use of cannabis as a significant risk factor for schizophrenia and schizophrenia-like psychotic illness.”

Mar. 2005