25 per cent of new cases of schizophrenia in the UK may be due to cannabis

If cannabis causes schizophrenia - and that remains in question - then by 2010 up to 25 per cent of new cases of schizophrenia in the UK may be due to cannabis, according to a new study by Dr Matthew Hickman of the University of Bristol and colleagues, published in Addiction journal.

Substantial increases in both prevalence and incidence of the disease are forecast by the end of the decade, with increases in schizophrenia starting earlier among young men in particular.

The research study matches historic trends in cannabis use and exposure from a national population survey against estimates of new occurrences of schizophrenia in three English cities (Nottingham, Bristol and the London Borough of Southwark). The researchers assess what might happen to schizophrenia cases if we assume a causal link between cannabis use and onset of psychotic symptoms, an association widely recognised by some psychiatrists and researchers and considered recently by the Advisory Council on the Misuse of Drugs.

Exposure to cannabis grew fourfold over the thirty years to 2002, and its use among under-18s by 18-fold, say the researchers. If cannabis use causes schizophrenia, these increases in its use would lead to increases in overall schizophrenia incidence and prevalence of 29 per cent and 12 per cent respectively, between 1990 and 2010. (Incidence is defined as the frequency of new occurrences; and prevalence is the percentage of the population affected by the disease.)

Model projections suggest that if the association is confined to heavy cannabis users only, then approximately 10 per cent of schizophrenia cases may be due to cannabis by 2010. However, assuming an association between onset of the disease and both light and heavy users, then approximately one-quarter of new cases could be due to cannabis.

John Macleod, co-author and academic GP, said: "We need to remember that our study does not address the question whether cannabis causes schizophrenia: this remains unclear."

Matthew Hickman, lead author of the study, added: "The challenge now is to improve our data on schizophrenia occurrence to see whether the projected increase occurs. This will tell us more about how important cannabis is as a cause of schizophrenia."