Certain politicians and other public figures have made much out of recent research that has found an association between the use of cannabis and the development of schizophrenia, but very little is known about the nature of this relationship.
According to an article published in the journal Addiction, even if cannabis use causes schizophrenia, between 2,800 and 7,700 heavy cannabis users would need to be prevented to prevent one case of schizophrenia.
The study was conducted by Matt Hickman, Peter Vickerman, John Macleod, Glyn Lewis, Stan Zammit, James Kirkbride and Peter Jones.
The estimates are based on data from studies conducted in Wales and England that investigated the incidence of schizophrenia, rates of heavy and light cannabis use, and the risk that cannabis causes schizophrenia.
As Hickman and his colleagues noted, the estimates of how many cannabis users would need to be prevented to prevent one case of schizophrenia, “were considerably high, even for young people who have the highest rates of schizophrenia, ranging for men aged 20–24 from 2800 for heavy cannabis users to more than 10 000 for light cannabis users; and for women aged 20–24 from 7700 for heavy cannabis users to 29 000 for light cannabis users.”
In comparison, Hickman and his colleagues explain that 108 people need to receive appropriate statin treatment in order to prevent one heart disease death and only 8.5 people need to receive online self-help to reduce one case of alcohol drinking problems.
Although Hickman and his colleagues claim that these estimates are based on the best available evidence, they acknowledge that their estimates are limited by the scarcity of information about the relationship between cannabis use and schizophrenia.
Their estimates are based on the assumption that the association between cannabis use and schizophrenia can be entirely explained by cannabis use and not other factors; an assumption which they acknowledge is very unlikely.
The study suggests that even if cannabis use is a risk factor for the development of schizophrenia, it is only a very slight risk and probably not a public health concern.
Hickman, M., Vickerman, P., Macleod, J., Lewis, G., Zammit, S., Kirkbride, J. & Jones, P. (2009) If cannabis caused schizophrenia—how many cannabis users may need to be prevented in order to prevent one case of schizophrenia? Addiction, Vol 104: 1856–1861.