Study leader Dr. Russ Callaghan, a scientist at Toronto's Center for Addiction and Mental Health, and colleagues also found heavy methamphetamine users might have an elevated risk of developing schizophrenia.
"We found that people hospitalized for methamphetamine dependence who did not have a diagnosis of schizophrenia or psychotic symptoms at the start of our study period had an approximately 1.5- to three-fold risk of subsequently being diagnosed with schizophrenia, compared with groups of patients who used cocaine, alcohol or opioid drugs," Callaghan said in a statement.
Callaghan found the increased risk of schizophrenia in methamphetamine users was similar to that of heavy users of marijuana.
The researchers examined California hospital records of patients admitted from 1990 to 2000 with diagnosis of dependence or abuse for several major abused drugs: methamphetamine, marijuana, alcohol, cocaine or opioids.
"We really do not understand how these drugs might increase schizophrenia risk," said Dr. Stephen Kish, also of CAMH. "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."
The findings were published in the American Journal of Psychiatry.