Meth and marijuana users are 9 times more likely to develop schizophrenia than hospitalized patients with no substance abuse history.
Researchers in Japan have previously linked meth use to persistent schizophrenia-like psychosis, but until now, scientists in the United States had discounted the theory.
A Study of 42,412 Meth Users
Consequently, researchers at the Toronto Centre for Addiction and Mental Health analyzed records of patients admitted to California hospitals between 1990 and 2000 who were diagnosed with dependence or abuse of methamphetamine, marijuana, alcohol, cocaine or opioids. There were 42,412 people in the methamphetamine group and 23,335 in the marijuana group. None had ever been diagnosed with schizophrenia or psychotic symptoms before hospitalization. Their records were compared to a control group of patients who were hospitalized for appendicitis.
They found that heavy meth use was significantly linked to an increase in risk of developing schizophrenia and they confirmed previous research that linked heavy marijuana use to schizophrenia-like psychosis.
Risk Higher in All Drug Users
Compared to the control group, meth and marijuana users were 9.37 times more likely to later be diagnosed with schizophrenia. Compared to alcohol, cocaine and opioid users, meth and marijuana users were 1.46 to 2.81 times more likely to develop schizophrenia.
The risk of schizophrenia was higher in all drug users than in the appendicitis group.
The researchers said they do not understand how meth and marijuana work to increase the risk of schizophrenia.
"Perhaps repeated use of methamphetamine and cannabis in some susceptible individuals can trigger latent schizophrenia by sensitizing the brain to dopamine, a brain chemical thought to be associated with psychosis," Dr. Stephen Kish said in a news release.
Callaghan, RC, et al. "Methamphetamine Use and Schizophrenia: A Population-Based Cohort Study in California." The American Journal of Psychiatry 8 November 2011. Related Information: