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.
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.
"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 labeled 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 labeled 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 f
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