Based on the analysis of Danish researchers, cannabis is now by far the highest non-genetic risk factor for schizophrenia, a devastating mental illness.
Schizophrenia’s best-known symptoms are paranoia and hallucinations, but the illness also damages motivation and even reduces overall intelligence. And people with schizophrenia are at high risk of committing violence.
The study suggests the United States, which has much higher cannabis use than Denmark, could already be seeing a rise in schizophrenia cases. But since the United States does not count new schizophrenia diagnoses, knowing for certain is nearly impossible.
The link between problematic cannabis use and the illness has risen dramatically in the last 50 years. Over the same period, cannabis has become far more powerful, with much higher levels of THC – the chemical responsible for its psychoactive effects.
The researchers could track the changes because Denmark’s national health care system lets them see all new diagnoses of both schizophrenia and cannabis use disorder – or marijuana addiction. They examined how many people with the disorder were later diagnosed with schizophrenia. Then they adjusted for other factors known to cause schizophrenia, such as a family history of mental illness.
The study was massive, covering the health records of almost 7 million people.