The assertion that cannabis use can cause schizophrenia is not borne out by the evidence, says Matthew Hill.
The 1936 film Reefer Madness depicted cannabis as a drug that provoked uncontrollable insanity, leading to manslaughter, suicide and attempted rape. This was a ridiculous characterization of the effects of cannabis, but there is a long history of associating the drug with psychotic disorders. In research terms, the first evidence came from a 1987 study, which found that Swedish conscripts had an increased risk of developing schizophrenia if they had consumed cannabis more than 50 times in their life. This finding has been replicated, implying, at the very least, an intricate relationship between cannabis use and schizophrenia. The nature of this relationship is still a matter of debate and is not as clear as some researchers or policymakers would suggest.