Bloomberg News
Young adults who used marijuana as teens were more likely to develop schizophrenia and psychotic symptoms, including hallucinations and delusions, than those who didn't, an Australian study has found.
Those who used the drug for six or more years were twice as likely to develop a psychosis such as schizophrenia or to have delusional disorders than those who never used marijuana, according to research released online by the Archives of General Psychiatry. They were also four times as likely to score high on a list of psychotic-like experiences.
The findings build on previous research and show that marijuana use isn't as harmless as some people think, lead study author John McGrath said Sunday. The authors said the study was the first to look at sibling pairs to discount genetic or environmental influence and still find marijuana linked to later psychosis.
"This is the most convincing evidence yet that the earlier you use cannabis, the more likely you are to have symptoms of a psychotic illness," said McGrath, a professor at the Queensland Brain Institute in Brisbane, Australia. "The message for teenagers is, if they choose to use cannabis, they have to understand there's a risk involved."
Marijuana, produced from the cannabis plant, can be smoked or ingested. Though its recreational use is illegal in the United States, about 14 states allow for medical use, which advocates say can ease nausea for cancer patients undergoing chemotherapy or help stimulate the appetites of AIDS patients.
McGrath said his researchers were looking for causes of schizophrenia.
The researchers looked at 3,801 young adults born in Brisbane from 1981 to 1984. At the 21-year follow up, the participants, whose average age was about 20, were asked about marijuana use. The researchers also measured whether they had psychotic symptoms.
Of the 1,272 participants who had never used marijuana, 26, or 2 percent, were diagnosed with psychosis. Of the 322 who had used marijuana for six or more years, 12, or 3.7 percent, were diagnosed with the illness. The study showed 65 people overall diagnosed with psychosis.
Researchers found that those who used marijuana the longest were four times more likely than those who didn't to have the highest scores derived from a list of psychotic-like experiences. McGrath said two of the questions posed to participants were "Do you ever feel as if you are possessed by someone or something else?" and "Do you ever feel as if other people can read your mind?" Even those who used marijuana for less than three years still had an increased risk of scoring higher than those who had not, McGrath said.
"Apart from the implications for policy makers and health planners, we hope our findings will encourage further clinical and animal-model research to unravel the mechanisms linking cannabis use and psychosis," the study authors wrote.
About 18 percent of participants said they used marijuana for three or fewer years, 16 percent said they used it for four to five years, and 14 percent used it for six or more years.
Researchers don't know when symptoms emerged or how much marijuana study participants used over their lives, McGrath said. Those in the study were interviewed at the ages of 14 and 21, so the symptoms emerged between those two study periods, he said.
Among 228 sibling pairs, those who didn't use marijuana reported fewer psychotic-like delusions compared with those who did, the authors said: a difference that is statistically significant and reduces the likelihood that the psychotic problems were caused by genetics or environment.
