Canadian researchers have found that heavy methamphetamine users may have a higher risk of developing schizophrenia. In the first worldwide study of its kind, scientists from Toronto's Centre for Addiction and Mental Health (CAMH) found evidence to support the conclusion that there may be a link between long-term heavy methamphetamine use and cannabis use in triggering latent schizophrenia. While Japanese clinical investigators had long suggested that exposure to methamphetamine might cause a persistent schizophrenia-like psychosis, such a possibility has been discounted in Western literature. The CAMH scientists, in an effort to investigate any relationship between drug use and the development of schizophrenia later, conducted a large-scale study of drug users who were initially free of schizophrenia. The researchers analyzed data from California in-patient hospital discharge records for the decade-long period from 1990 through 2000. They were looking for patients who had been admitted with a diagnosis of abuse or dependence on several major drugs of abuse, including methamphetamine, cocaine, alcohol or cannabis. Also included in the study as a control group were patients admitted with appendicitis and no drug use history. Excluded from the study were patients who were dependent upon or abused more than one drug or those that had a diagnosis of schizophrenia or drug-induced psychosis during their initial hospitalization. The data showed that methamphetamine users had a significantly higher risk of schizophrenia than the appendicitis control group, and the cocaine, opioid and alcohol groups. But meth users' risk of schizophrenia was not significantly different from that of the cannabis group. The study further found that the schizophrenia risk was higher in all the drug groups analyzed than the appendicitis group. Dr. Stephen Kish, senior scientist and CAMH's Human Brain Laboratory head said scientists really don't understand how these drugs might increase schizophrenia risk. "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." While the potential link has been identified, researchers caution that the results need to be confirmed in additional research that involves long-term follow-up studied of methamphetamine users. The results of the CAMH study are published in AJP in Advance, the advance edition of the American Journal of Psychiatry, which is the official journal of the American Psychiatric Association.