September 29, 2009

What Proportion Of Psychotic Illness Is Due To Cannabis?

In this week's PLoS Medicine, a team of researchers from Australia and the US, led by Louisa Degenhardt at the University of New South Wales, Sydney, makes the case for estimating the role that cannabis has worldwide as a risk factor for psychosis.

This estimation, says the team, will give an idea of how much impact cannabis has upon public health globally. The information in turn could be valuable for guiding health policymakers in deciding about health policies, services, and research.

The global impact of different diseases and risk factors upon population health is estimated by a high profile international research initiative called the Global Burden of Disease Project ( Some of the risk factors that the project assesses are smoking, high blood pressure, obesity, and alcohol use. But in the past the project has not examined cannabis as a risk factor, say Degenhardt and colleagues, because of concerns that the evidence linking cannabis use to psychosis is too weak.

Degenhardt and colleagues examine the studies that have shown a link between using cannabis and developing psychotic illnesses such as schizophrenia. Based on the strength of this evidence, and on the fact that cannabis use is a potentially preventable exposure, they argue that the Global Burden of Disease Project should include cannabis as a risk factor. The authors review lines of evidence which suggest that cannabis may be a particular risk for those vulnerable to developing the illness. They propose to model multiple possible relationships between cannabis and psychosis, including models of poorer outcomes for those who have developed the disorder.

Researchers in Australia, for example, included cannabis in their national study of the impact of risk factors and diseases upon population health. In estimating the impact of cannabis, the researchers assumed that the evidence was good enough to show a link between cannabis use and psychosis, suicide, and car crashes. "Even after assuming that these relationships were causal," say Degenhardt and colleagues, "cannabis was not a major contributor to disease burden in Australia, accounting for 0.2% of all disease burden, which amounted to 10% of the total burden attributable to all illicit drugs."

"These estimates are important for public policy purposes," they say, "because failure to make them allows untested estimates to be offered in public policy debate."

The authors argue that if the international community does not estimate the global impact of cannabis use, there will be important consequences. "There will be a reduced public health, policy, or research imperative, since there will be no estimated burden."
On the other hand, they say that "if we do attempt to estimate burden, future work will examine the accuracy of our estimates and refine them as evidence accumulates. Debates may emerge and (hopefully) improvements made as new evidence supports or challenges the assumptions made.''

Funding: This work was given funding support from the Australian Government Department of Health and Ageing. LD is the recipient of an Australian National Health and Medical Research Council (NHMRC) Senior Research Fellowship. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing Interests: The authors have declared that no competing interests exist.

Citation: Degenhardt L, Hall WD, Lynskey M, McGrath J, McLaren J, et al. (2009) Should Burden of Disease Estimates Include Cannabis Use as a Risk Factor for Psychosis? PLoS Med 6(9): e1000133. doi:10.1371/journal.pmed.1000133


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