Are Smoking Pot and Psychosis Linked?

July 26, 2007 -- Smoking cannabis, or marijuana, as a youth could boost the risk of developing a psychotic illness later in life by about 40%, according to a new analysis of published studies conducted by British researchers.

The more than 40% 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, MD, PhD, 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.

Study Details

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."

Use Patterns

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% of youths aged 16 to 24 say they use cannabis on a monthly basis.