Marijuana Smoking Tied
to Depression, Schizophrenia
Excerpt By Amy Norton, Reuter's Health

NEW YORK (Reuters Health) - Young people who frequently smoke marijuana may be more likely to later develop depression, anxiety and even schizophrenia, the results of three studies released Friday suggest.

The studies do not establish a definite cause-and-effect relationship. However, researchers say the argument that people prone to mental illness may simply be more likely to use marijuana does not appear to entirely explain the findings.

Whether marijuana is really the culprit behind these mental health problems has long been difficult to establish. In the case of schizophrenia, for example, it has been suggested that people who are in the early stages of developing the disorder may choose to "self-medicate" with drugs like marijuana.

All three reports are published in the November 23rd issue of the British Medical Journal.

One of the studies followed more than 50,000 Swedish men drafted for military service between 1969 and 1970. Researchers found that those who said they had used marijuana, and no other drugs, were more likely than non-drug users to be diagnosed with schizophrenia over the next 27 years.

And the greater the marijuana use, the higher the risk, the study's lead author, Dr. Stanley Zammit, told Reuters Health.

Zammit's team found that among men who reported marijuana as their sole drug of abuse, those who had used it at least 50 times at the start of the study had a nearly seven-times greater risk of schizophrenia. The link remained when the researchers factored in the men's mental health evaluations at the study's outset. They also considered factors such as the men's IQ scores and "personality" traits.

"We cannot be certain that this increase in risk is due to cannabis itself, although we tried to take into account other factors that we felt might explain the association," said Zammit, a researcher at the University of Wales College of Medicine in Cardiff, UK.

Still, he added, considering the fact that schizophrenia risk increased in tandem with marijuana use, "I think the most likely explanation for the increased risk is that of cannabis use."

Past research has suggested that heavy pot smoking may eventually impair memory and learning, as well as mental health. In the other two new studies, researchers found evidence that heavy marijuana use in adolescence may raise the odds of depression, anxiety or schizophrenia-like symptoms later on.

In one, Australian researchers found that among girls in 44 secondary schools, those who smoked pot every day were five times more likely to develop signs of depression or anxiety over the next 7 years. On the other hand, students with depression or anxiety to begin with were not more likely to start using pot.

The third study looked at 759 New Zealand teens followed from birth to age 26. Researchers found that those who had started using pot by age 15 or 18 were more likely than non-users to develop "schizophrenia symptoms" by the time they were 26--regardless of whether they had shown psychotic symptoms when they were evaluated at age 11.

In addition, earlier pot smoking--by age 15--was associated with a greater risk of schizophrenia symptoms, compared with relatively later use of the drug.

"Most young people use cannabis in adolescence without harm," write Dr. Louise Arseneault, of King's College in London, and her co-authors. Still, they conclude, their findings suggest that when it comes to "psychologically vulnerable" teens, marijuana use "should be strongly discouraged."

Zammit pointed out that there is a biological basis to believe that heavy marijuana use could promote mental disorders because the drug acts on receptors found throughout the central nervous system.

But he also noted that with schizophrenia, the underlying cause is complex and not fully understood. "We know that there is no single cause of schizophrenia," Zammit said, "but only lots of factors that increase someone's risk of developing this illness."

If marijuana use does in fact help promote schizophrenia, according to the researcher, the odds that any one user would develop the disorder would still be quite low.

However, Zammit said, "people who use cannabis should be made aware of this possible risk, especially if they use a lot."

SOURCE: British Medical Journal 2002;325:1183-1184, 1195-1198, 1199-1201, 1212-1213.

Reference Source 89

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