THURSDAY, Nov. 21, 2002 (HealthDayNews) -- As many as one in seven cases of schizophrenia could be prevented by eradicating marijuana.
That's the contention of a new study of Swedish soldiers that found smoking pot increases the risk of the psychiatric disorder by about 30 percent.
The Swedish study is one of three reports in this week's British Medical Journal linking marijuana to emotional problems, including depression and anxiety, as well as schizophrenia.
Some scientists believe marijuana doesn't cause psychiatric illness. Rather, they argue, people smoke it as a way to self-medicate. However, the authors of the Swedish study say their evidence suggests that now seems unlikely.
"It's not as good an explanation than the possibility that cannabis itself causes schizophrenia," says Dr. Stanley Zammit, a psychiatrist at the University of Wales College of Medicine in Cardiff and lead author of the new paper.
Zammit and his colleagues compared schizophrenia rates and marijuana use among more than 50,000 Swedish conscripts who participated in a 1969-1970 survey. Of those, 362, or about 0.7 percent, had been hospitalized with the disorder by 1996.
Of the 11 percent of the entire group that admitted ever trying marijuana, 73, or 1.4 percent, went on to be hospitalized for schizophrenia. The odds of having the disease grew as pot use climbed, reaching nearly a sevenfold increase in the men who used it 50 times or more but tried no other illegal drugs, the study says.
The effect was strongest among soldiers who developed schizophrenia within five years of entering the military. It held after the researchers accounted for use of alcohol and other drugs.
"You would expect that if there was a common reason for using substances, that would have eliminated" the influence of marijuana, Zammit says.
The second study in the British Medical Journal, of 759 New Zealanders, found that those who started smoking pot by the time they were 15 years old had quadruple the risk of non-users of being diagnosed by age 26 with schizophrenia and related disorders. That risk didn't hold for people who began taking the drug at age 18 or later. The study also found that 10 percent of the young smokers went on to develop psychosis, compared with 3 percent of the rest of the study group.
Louise Arseneault, a psychiatrist at King's College, London, and a co-author of the study, says marijuana use "predisposes" children to schizophrenia later in life.
"It's part of a complex group of causes. You don't need to smoke cannabis to have schizophrenia," Arseneault says. "A lot of people have it who didn't smoke, and a lot smoke but don't get it."
In the third study, scientists in Australia found that teens who smoked marijuana at least once a week were twice as likely as less frequent users to suffer depression or anxiety over the next seven years. Girls who used the drug every day had between five and six times the rate of these conditions as non-users.
The study also found that teens with emotional problems at the beginning of the study weren't more likely to take up marijuana in the future -- suggesting that self-medication wasn't a factor.
Michael Lynskey, a psychiatrist at Washington University in St. Louis and a co-author of the Australian study, says he "wouldn't want to make a definitive statement either way" about whether marijuana causes psychosis or depression. Even a doubling of the risk of depression is considered a relatively small increase, he says. So the effect of marijuana is probably modest.
However, research shows that people who take THC, the active ingredient in pot, to control nausea are more prone to depression, Lynskey says. He considers that "strong evidence" that marijuana can play a role in depression.
What To Do
To find out more about schizophrenia, visit the National Alliance for Research on Schizophrenia and Depression. For more on the health risks of marijuana, see Harvard University.
