New findings may fuel those worries. At the World Psychiatric Association’s World Congress in Berlin on October 9, Hannelore Ehrenreich of the Max Planck Institute of Experimental Medicine presented results of a study of 1,200 people with schizophrenia. The investigation analyzed a wide range of genetic and environmental risk factors for developing the debilitating mental illness. The results—being submitted for publication—show people who had consumed cannabis before age 18 developed schizophrenia approximately 10 years earlier than others. The higher the frequency of use, the data indicated, the earlier the age of schizophrenia onset. In her study neither alcohol use nor genetics predicted an earlier time of inception, but pot did. “Cannabis use during puberty is a major risk factor for schizophrenia,” Ehrenreich says.

Other studies, although not all, support the thrust of Ehrenreich’s findings. “There is no doubt,” concludes Robin Murray, a professor of psychiatry at King’s College London, that cannabis use in young people increases the risk of developing schizophrenia as an adult.