Young people who use marijuana (cannabis) are at increased risk of suffering hallucinations, delusions or other reality-distorting “psychoses.” And the more time that’s passed since first use, the higher the risk.

The findings from a study by Dr. John McGrath, of the Queensland Centre for Mental Health Research in Wacol, Australia, and colleagues confirm previous smaller studies that have suggested that pot smoking may be linked to mental illness.

The study, appearing in the Archives of General Psychiatry, involved roughly 3,800 people born in Brisbane between 1981 and 1984, who were followed up at age 5, 14 and 21 years. When they were 20 years old on average, researchers asked them about marijuana use and assessed their mental health.

About 18 percent of the group said they smoked marijuana for three or fewer years, 16 percent admitted smoking pot for four to five years and 14 percent for six or more years. A total of 65 participants had been formally diagnosed with a psychotic disorder such as schizophrenia, and 233 had hallucinated at least once.

Compared with those who had never used marijuana, those who first smoked marijuana when they were 15 or younger were twice as likely to be diagnosed with schizophrenia or other psychotic illness, four times as likely to suffer delusions, and nearly three times as likely to suffer hallucinations.

The association between marijuana use and psychotic symptoms remained true after the researchers analyzed 228 sibling pairs separately. “Our study is the first to look at siblings,” McGrath noted. That allowed the team to limit the effect of other factors. “Put bluntly, it makes the findings stronger and more convincing,” he said.

So does pot smoking causes mental illness? It’s not that simple, according to McGrath and colleagues. The young adults in their study who suffered hallucinations early in life, and were therefore more vulnerable to psychosis, were also more likely to report using marijuana early in life.

“Young people,” McGrath told Reuters Health by email, “need to be educated about the risks of psychosis if they start using cannabis at a young age. Adolescence and young adulthood are vulnerable periods for the development of mental illness, and cannabis seems to increase the risk for some vulnerable people.”

More research, the authors conclude, is needed to determine conclusively whether or not smoking pot causes mental illness.

SOURCE: Archives of General Psychiatry, online February 28, 2010.