Young adults who used marijuana as teens were more likely than those who didn’t to develop schizophrenia and psychotic symptoms including hallucinations and delusions, an Australian study 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 shows that marijuana use isn’t as harmless as some people think, lead study author John McGrath said yesterday in an e-mail. The study was the first to look at sibling pairs to discount genetic or environmental influence and still find marijuana linked to later psychosis, the authors said in the study.
“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, in a statement. “The message for teenagers is: if they choose to use cannabis they have to understand there’s a risk involved.”
Smoked or Ingested
Marijuana, produced from the cannabis plant, can be smoked or ingested. Its recreational use is illegal in the U.S. About 14 U.S. states have laws allowing for medical use of marijuana, which advocates say can ease nausea for cancer patients undergoing chemotherapy or help stimulate the appetites of AIDS patients.
Researchers in the study were looking for causes of schizophrenia, McGrath said.
The researchers included 3,801 young adults who were 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 those in the study had psychotic symptoms.
Of the 1,272 participants who had never used marijuana, 26 or 2 percent were diagnosed with psychosis. Of the 322 people who had used marijuana for six or more years, 12 or 3.7 percent were diagnosed with the illness. Overall, 65 people were diagnosed with psychosis, according to the study.
The researchers also 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. Two of the questions posed to study participants, according to McGrath, asked: “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?”
McGrath said that even those who used marijuana for fewer than three years still had an increased risk of scoring higher than those who had not.
“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 those in the study 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.
The study also showed that among 228 sibling pairs, those who didn’t use marijuana reported fewer psychotic-like delusions compared with those who used cannabis. That difference was statistically significant and reduces the likelihood that the psychotic problems were caused by genetics or environment, the authors said.
The study also looked at siblings who both smoked pot and found that those who smoked it longer had a higher score for psychotic-like delusions than the other sibling who smoked for a shorter time.
About 2.4 million American adults have schizophrenia, according to the National Institutes of Health. The disorder often appears in men in their late teens and early 20s, while in women it generally strikes in their early 20s or 30s. It is associated with delusions, hallucinations and disordered thinking, McGrath said. It is unclear what causes the disease.
The study was funded by the National Health and Medical Research Council of Australia.
By Nicole Ostrow–Editors: Donna Alvarado, Andrew Pollack, Feb 2010