Smoking marijuana might trigger an earlier onset of psychosis, a new analysis of previously published research suggests.
Among people who developed a psychotic illness, those who had smoked pot developed the illness almost three years earlier than those who hadn't, the Australian researchers said, and they speculated that marijuana may actually cause psychosis in some patients. Their report will appear in the Archives of General Psychiatry.
But Michael Rice, a psychiatric nurse practitioner at the University of Nebraska Medical Center in Omaha, isn't convinced that smoking pot causes psychosis and said there are still several other theories that could explain the relationship.
One hypothesis is that schizophrenia may be due to an inflammation in the brain, perhaps due to a virus, and that people with pre-symptomatic schizophrenia gravitate towards pot because it has an anti-inflammatory effect, said Rice, who is also associate director of Nebraska's Behavioral Health Education Center.
Marijuana used by millions
"People may be naturally seeking out some substance that tries to reduce the inflammation and the inflammation itself may be related to the psychosis," he explained.
And some neurological and cognitive changes that can predict the onset of schizophrenia later can be detected as early as six or seven years of age, he added.
Millions of people use marijuana regularly, and most started smoking in their teens. It is the third most widely used addictive substance after tobacco and alcohol, the study authors noted.
Earlier studies had found a link between psychosis and marijuana use, but there's been some controversy over whether the drug actually causes psychosis. Earlier studies didn't necessarily look at the timing of onset of psychosis in relation to when people had started using marijuana.
This meta-analysis (where researchers combine data from numerous related studies) looked at 83 peer-reviewed articles involving almost 8,200 individuals who had used marijuana or other drugs and almost 14,400 individuals who had not.
People who smoked pot developed psychosis 2.7 years earlier than people with psychosis who did not smoke pot.
Marijuana and psychosis
Study author Dr Matthew Large, a senior staff psychiatrist at Prince of Wales Hospital in Sydney, said, "Heavy cannabis use starting at a young age carries a very much greater risk than modest use as an adult."
In general, people who used illicit drugs developed their psychosis two years earlier than those who didn't. Alcohol use didn't seem to speed the onset of psychosis.
According to Large, about one in 20 people who start using marijuana while in junior high school (early teens) will develop psychosis, while only one in 50 people who use as adults will.
"Given the right genetics, if you smoke pot, it will trigger illness," Rice said. Still, he cautioned that the picture is more complicated than that.
"Schizophrenia is not a single disease. It's 25 to 30 illnesses with similar features, which makes it difficult to come to a single conclusion," he said. "Schizophrenia is the cancer of mental illness."
Another expert agreed.
Cannabis a risk factor for schizophrenia
"Folks in the field appreciate that schizophrenia and other psychotic disorders are really a wide range of various etiologic manifestations," explained Dr Kathryn Kotrla, associate dean at the Texas A&M Health Science Center College of Medicine in Round Rock. "We don't think that every individual labelled with a psychotic disorder arises from the same cause."
"If there's a plateau and a cliff, when you fall off the cliff, you get labelled as having schizophrenia but there are many steps up to that cliff and each step a risk factor," she said. "We need to think about cannabis as another potential risk factor, another step towards the cliff."
Nevertheless, the study's authors emphasised a need for public health messages dissuading people - especially young people - from using pot.
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