The more than 40 percent increase in risk applies to those who have ever used the drug, and the risk rises even more with frequent use, according to Stanley Zammit, M.D., Ph.D., clinical lecturer in psychiatric epidemiology at Cardiff University and the University of Bristol in the U.K., a study co-author.
"People who have ever used cannabis, on average, have about a 40% increased risk of developing psychotic illness later in life compared with people who have never used cannabis," he tells WebMD.
"People who used it on a weekly or daily basis had about a 100% increased risk, or twofold." Even so, he adds, "the risk is still relatively low."
But as Zammit and his colleagues note in the new report, scheduled to appear in the July 28 issue of The Lancet, there is enough evidence of a marijuana-psychosis link that they believe policymakers need to provide the public with information.
The report drew protests and skepticism from representatives of The National Organization for the Reform of Marijuana Laws, who questioned the validity of the findings.
Zammit and his colleagues pooled the results of 35 published studies on marijuana use and mental health effects, including psychotic effects such as schizophrenia (in which people may hear voices or hallucinate) or affective problems such as depression and anxiety. They analyzed the results of all the studies, a method known as a meta-analysis.
The increased risk of psychosis with marijuana use persists, Zammit's team found, independently of the transient intoxication effects of the drug and independently of what they call "confounding factors," such as existing mental health problems or other drug use. "We can't be sure it is causal," he says of the association. "[But] studies find an association rather consistently."
Still, he tells WebMD, "It's always possible people who use cannabis may be different [in some way] than those who don't."
The researchers also looked at the association between marijuana use and depression and anxiety but found that the evidence is "less strong than for psychosis but is still of concern."
In the U.S., marijuana is the most widely used of various illicit drugs, according to the University of Michigan Monitoring the Future Survey. About 6.8% of middle school and high school students used marijuana in 2005, down from 7.6% the previous year, according to the annual National Survey on Drug Use and Health, a federal report.
In the U.K., Zammit estimates, about 15 percent of youths aged 16 to 24 say they use cannabis on a monthly basis.
In an accompanying comment, two scientists from Copenhagen University Hospital echo Zammit's belief that "there is a need to warn the public of these dangers, as well as to establish treatment to help young, frequent cannabis users."
In an editorial in the same issue, Lancet editors note that the publication ran an oft-quoted editorial in a 1995 issue stating that "the smoking of cannabis, even long term, is not harmful to health." Now, the editors note, research published in the interim, including the meta-analysis, has triggered a change in their thinking, with them now stating that cannabis use "could increase the risk of psychotic illness" and that more research is needed on any link with depression and anxiety.
If the association exists between marijuana use and psychotic illness, "we would have seen the negative effects they were warning about if they were significant," says Paul Armentano, a senior policy analyst for the National Organization for the Reform of Marijuana Laws (NORML), based in Washington.
Most Western cultures, he tells WebMD, have witnessed "an explosion among marijuana use among adults and young people.
"Where is the explosion in cannabis-related mental illness?" he asks. "The paper says, 'You are right, we haven't seen it. Maybe it is a delayed reaction.'"
Armentano argues the rise in mental illness would have already occurred if the link exists.
Armentano also wonders if the psychosis may have come first, before the marijuana use, for some people. In the paper, the authors note that such reverse causation is not likely for psychosis but that the studies of marijuana and depression did not adequately address the possibility of reverse causation.
Politics in the U.K. may be driving the effort to analyze an association between marijuana and mental illness, Armentano tells WebMD. Prime Minister Gordon Brown has been quoted in the British press as saying he has never used cannabis, even as cabinet ministers tell about their cannabis-filled younger days. In 2004, the U.K. downgraded cannabis to a class C drug, reducing penalties for possession, production, and supply.
Now, the Advisory Council on the Misuse of Drugs in the U.K. will look at evidence for harms caused by cannabis and discuss whether the drug should be relabeled, perhaps as a class B drug of misuse, with stiffer penalties for possession.
"The article is worth paying attention to," Bruce Spring, M.D., an assistant professor of clinical psychiatry at the University of Southern California Keck School of Medicine, Los Angeles, an expert familiar with the study but not involved in it.
"It certainly gives cause for concern," Spring says of the findings about marijuana and psychosis risks. Still, he says, the overall risk is relatively low, statistically speaking.
"In general, the overall risk of someone getting a psychotic illness is about 3 percent," he says. "Now what this study is saying is that that 3% risk is increased by 40 percent [or more]," he says. So the risk with marijuana use would rise to 4.2 percent.
Put another way: In a group of 100 people, three would be expected, statistically speaking, to develop a psychotic illness such as schizophrenia or bipolar disorder with psychosis. "When you factor in the marijuana study, one or two more, depending on how often they use it, will have psychotic illness," Spring says.
Says Spring, "I would tell people there is now some pretty good evidence that smoking marijuana can have some harmful consequences, and they are putting their future well-being at risk [if they smoke marijuana]. The more you smoke, the greater the risk, according to this study."
Zammit, the study co-author, adds, "I think the important message is to be aware of these risks." Those who have other risk factors for psychotic illness, such as a family history, might want to pay closer attention, he tells WebMD.
By Kathleen Doheny
Reviewed by Louise Chang
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