Teenage marijuana smokers are at higher risk for developing psychoses -- such as schizophrenia, hallucinations and delusions -- compared with those who don't smoke marijuana, a new study says.
The study tracked 3,800 people born from 1981 to 1984, according to researchers at the Brain Institute at the University of Queensland in Australia. They asked the test subjects at ages 14 and 21, and their mothers, about their mental health record and whether they had used any drugs, specifically cannabis. Fourteen percent acknowledged using marijuana for six or more years.
"We looked at the association between how old they were when they first started to use cannabis. And then, on the other hand, we looked at how their mental health was and, in particular, whether they had psychotic disorders or isolated symptoms, such as hallucinations or delusions. And indeed we found a highly significant relationship," said psychiatrist John McGrath, a professor at the Brain Institute, the study's lead researcher.
"For those who started using cannabis when they were 14 or 15, they had about a two-fold risk of schizophrenia."
"The quality of the evidence is really mounting up," McGrath said. "There have been several studies now, which all point in the same direction, and there's consistent and robust evidence that there's an association between cannabis use and increased risk of psychosis."
The University of Queensland study did not consider how much marijuana the teens used, only their age when they started using.