Article: Psychotic symptoms more likely with cannabis / New Scientist, 1 Dec 2004

http://www.newscientist.com/news/news.jsp?id=ns99996745
"Using marijuana in adolescence and early adulthood can cause psychotic symptoms later in life, a new study suggests. The risk of developing these symptoms is �moderate�, say researchers, though is higher in people with a pre-disposition to psychosis."