Does Cannabis Cause Psychosis?A study of trends in cananbis use and and psychosis in Elgnand 1995-2003
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Dr. R.D. Newcombe, Liverpool John Moores University, Liverpool, England; August 2004
Introduction. Recent literature reviews have concluded that the notion of cannabis-induced psychosis lacks conceptual and empirical validity. This paper focuses on 'true' cannabis psychosis (CP) - a unique mental disorder arising from intoxication by cannabinoids, persisting beyond their metabolisation, and occurring without predisposition to psychosis.
Methods. A documentary research study was designed to evaluate the hypothesis that cannabis use (CU) is a cause of psychosis " either general (schizophrenia) or unique (true CP). This hypothesis generated two testable predictions: (1) trends in CU and CP should be positively correlated, and (2) the demographic and treatment characteristics of CP cases should be different from the profiles of both cases of schizophrenia and other cannabis-related mental and behavioural disorders (MBDs) defined by ICD-10 " notably acute cannabis intoxication (ACI) and harmful cannabis use (HCU). National official statistics were collected on (a) the annual number and characteristics of diagnosed cases of schizophrenia and cannabis-related MBDs in England from 1998/99 (1995/96 for CP) to 2002/03 (Hospital Episode Statistics); and (b) the prevalence of past-year CU among 16-59s in England & Wales in the decade ending 2002/03 (British Crime Survey).
Findings. The annual rate of CP among English CUs was typically as low as one in 10,000. But although past-year CU climbed from 2.55 million (8.7%) in 1994 to 3.36 million (10.9%) in 2002/03, there were no clear trends in either schizophrenia (36,000-38,500 cases annually) or CP cases (280-380). Both predictions were disconfirmed. First, the correlation between CU and CP numbers over six comparison years was found to be small and non-significant. Second, although CP cases were clearly different from schizophrenia cases - including being a mean 15-years younger, and averaging one (rather than 4-5) months in-patient treatment " their profile was very similar to that of both ACI and HCU cases. Lastly, assessment of research evidence about cannabis consumption over the study period found no significant changes in relevant variables (eg. dosage, THC potency).
Conclusions. There was no support for the claim that cannabis use can cause psychosis, nor for a 'true' CP. Instead, CP cases were arguably misdiagnoses of extreme cases of ACI and HCU, and/or MBDs arising from other/multiple drug use. Urgent research is also needed into which ingredients of cannabis are responsible for any mental disorders which are attributed to its use, particularly since Britain's most popular product - Moroccan soap-bar/formula - is routinely adulterated with toxic chemicals.
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