People who smoke marijuana daily or weekly double their risk of developing a psychotic illness over their lifetime, according to a study published Thursday.
Among all cannabis users, including sporadic experimenters and habitual users, the lifetime risk of psychotic illness increased by 40%, the report said.
"It's not as if you smoke a joint and you're going to go crazy," said Richard Rawson, who directs the Integrated Substance Abuse Program at UCLA and was not involved in the study.
But he cautioned: "It's definitely not a good idea to use heavy amounts of marijuana."
The researchers found that the risk for psychotic illnesses did appear to increase with dose, suggesting that stopping marijuana use would decrease risk, said coauthor Dr. Stanley Zammit, a psychiatrist at Cardiff University and the University of Bristol in Britain.
Psychotic illnesses include schizophrenia and disorders with such symptoms as hallucinations or delusions.
Marijuana is the most commonly used illicit drug in the U.S., according to the federal government. In 2006, about 42% of America's high school seniors reported having tried marijuana at least once, according to an annual report funded by the National Institute on Drug Abuse.
Marijuana can cause psychiatric problems because it throws off the balance of neurotransmitters in the brain, Zammit said.
Previous studies have had difficulty untangling marijuana's role in psychiatric disorders. Smoking the drug could be a symptom of a disorder rather than a cause.
The study by Zammit and colleagues, published in the medical journal the Lancet, reanalyzed data from seven long-term studies on psychotic illnesses and marijuana involving 61,000 participants.
The researchers filtered out about 60 factors, such as preexisting mental illness and the use of other illicit drugs, and considered IQ and social class, to try to isolate the effect of marijuana, Zammit said.
Most of the studies that were analyzed indicated a range of increased risk for frequent users from 50% to 200%, with the average being about 100%, or double the risk, Zammit said.
The researchers also studied the relationship between marijuana use and mood disorders, such as depression and bipolar disorder. They analyzed 22 studies involving 52,000 participants.
The researchers found that any marijuana use increased the lifetime risk for mood disorders by about 40%, and weekly or daily use increased the risk by about 50%.
The mood disorder studies were less successful in filtering outside factors, so the increased risk may be unrelated to smoking marijuana, Zammit said.
Dr. Victor Reus, a psychiatrist at UC San Francisco who was not involved in this study, said he was unconvinced by Zammit's conclusions for both psychotic and mood disorders.
Too many outside factors contribute to the disorders, and the studies Zammit used were too vague to draw hard conclusions, he said.
"There's a limit to what you can do with the data that's in these studies," he said.