Using cannabis increases the risk of developing serious mental illness, researchers warn today.
A team of researchers led by Professor Jim van Os from Maastricht University in The Netherlands, looked at the rate of "subclinical" psychosis – that is, below the level needed for a clinical diagnosis.
They point out that cannabis is consistently linked with an increased risk for mental illness, but the direction of causation is unclear. Individuals with psychosis may use the drug to "self-medicate".
The team took figures from a population-wide database in Germany, with 1,923 participants, aged 14 to 24. Use of cannabis and psychotic symptoms were measured at the start, after three years, and after eight years.
Those who had begun to use cannabis in the first three years of the study were 90 per cent more likely to have psychotic symptoms by the end. Continued use raised the risk further, to more than double.
One in three cannabis users had psychotic symptoms after three years, compared to one in five non-users, they report in the British Medical Journal.
The authors report: "Cannabis use is a risk factor for the development of incident psychotic symptoms. Furthermore, in those with cannabis use at the start of the study, continued use over the study period increased the risk of persistent psychotic symptoms."
They add that there was no evidence for self-medication, as having psychotic symptoms did not predict later cannabis use.
In an editorial, Professor Wayne Hall from the University of Queensland, Australia, says that policies on cannabis "should be based not only on the harms caused by cannabis use, but also on the harms caused by social policies that attempt to discourage its use, such as criminal penalties for possession and use".
Hall, W. and Degenhardt, L. Cannabis and the increased incidence and persistence of psychosis. The British Medical Journal, 2011;342:d719.
Leave a Reply