Cannabis-related Schizophrenia May Be Set To Rise, Say Researchers

A new study projects what might happen to the incidence of schizophrenia if there is a relationship between marijuana use and schizophrenia. It's worth noting that even people who believe that there is a relationship, believe that the risk is low.

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.)