Toking up may not kill you, but it could trigger mental illness. People who use marijuana are 40% more likely to develop some form of psychosis than are those who just say no, according to a new analysis, and the risk soars for the heaviest users. The findings could put a new spin on the debate raging over legalizing the drug.
For many years, studies have hinted at a link between smoking cannabis and psychosis, a mental illness characterized by hallucinations and delusions that can lead to schizophrenia. A clear relationship has remained elusive, however, in part because researchers have used different methods and time frames to address the question.
Seeing the need for a systematic review of the evidence, a team of researchers from four British universities combed through 35 studies of children and adults from New Zealand, the United States, and Europe. All tracked groups of people initially free of mental illness over periods of up to 30 years and compared the incidence of new mental symptoms in cannabis users and nonusers.
The findings of the individual studies varied, but when the team, led by psychiatrist Theresa Moore of the University of Bristol, U.K., pooled the data and controlled for confounding variables, it found a 41% increase in psychosis among cannabis users. According to the authors, the psychosis risk for occasional users is roughly 3% to 4%, compared with a 2% to 3% lifetime risk in the general population. Among heavier users--people who smoked pot more than 100 times--the odds of developing psychosis increased an average of more than 200% compared to the general population. The risk of schizophrenia jumped as well, from 1% in the general population to roughly 2% in heavy users, the team reports in this week's issue of The Lancet. That risk is still relatively low, says co-author Stanley Zammit, a psychiatrist at Cardiff University in Wales, but it might be enough to prompt some users to cut back or stop smoking. The researchers also looked at the relationship between cannabis and anxiety, depression, and suicidal thoughts, but links between marijuana and these disorders were less consistent than for psychosis.
The authors agree that some uncertainty remains and concede that their analysis is based only on observational studies rather than on more reliable randomized controlled studies like those used to test new drugs. In addition, they note that the incidence of schizophrenia in the general population has not increased with rates of marijuana use, as would be expected if marijuana was a causal agent in the disease.
However, for many researchers, the review comes as confirmation of a long-recognized trend. The study "is an important ... contribution to the debate on the association between cannabis use and psychosis," says Nadia Solowij, a psychologist at the University of Wollongong in Australia. Neurobiologists have previously shown an overlap in the location and functioning of the parts of the brain sensitive to marijuana's active ingredient, THC, with regions implicated in psychosis and schizophrenia, she says, and the new findings help solidify the linkage on a clinical level.