Heavy methamphetamine users may be at a significantly higher risk for developing schizophrenia than others, according to researchers from Toronto's Centre of Addiction and Mental Health (CAMH). The researchers' findings-based on the world's first expansive study on long-term methamphetamine use's association to psychosis-have been published in advanced online in the American Journal of Psychiatry. Previous studies in Japan have suggested such a link between methamphetamine exposure and the development of chronic schizophrenia-like psychosis-yet Western literature had yet to consider the correlation, even though methamphetamine\/amphetamine-type stimulants are the second most widely used illicit drug worldwide. Most Western mental health experts believe that schizophrenia found among methamphetamine users is most likely already present yet undiagnosed, or that these types of persons are more susceptible to methamphetamine use. Other studies have identified a similar link between cannabis dependence and schizophrenia, but large-scale studies involving methamphetamine or amphetamine-type drugs and the mental disorder had never been investigated. Therefore, CAMH scientist Dr. Russell Callaghan and his colleagues conducted a large cohort study comparing the risk for schizophrenia among heavy methamphetamine users with the rest of the population. For their study, the researchers gathered data on tens of thousands of patients based on California inpatient hospital discharge records from 1990 to 2000. At the beginning of the study's time frame, the researchers identified 42,412 heavy methamphetamine users who did not have a prior history or current diagnosis of psychosis or dependence on other substances. Then, these patients' propensity for later developing psychosis were compared with that of patients identified as having addictions to other illicit substances (including alcohol, cocaine, opioids, and cannabis-with 23,335 cannabis users), as well as healthy patients who were treated for appendicitis. The rates of later developing schizophrenia were mapped based on subsequent hospital admissions with schizophrenia diagnoses among all the groups within the ten-year span. From their results, the researchers found the heavy methamphetamine users had anywhere from a 1.5 to 3.0-fold risk for later being diagnosed with schizophrenia when compared to heavy users of alcohol, cocaine, or opioids. According to the researchers, this heightened risk for schizophrenia among heavy methamphetamine users was parallel to that among heavy cannabis users. Additionally, the methamphetamine group had a significantly higher risk for later schizophrenia in comparison to the healthy appendicitis group (9.37 times more likely of subsequent diagnosis). In fact, all of the drug users had a noticeably higher risk of later schizophrenia than patients from the healthy group. Why methamphetamine and cannabis might increase the risk for schizophrenia remains unknown, but the researchers suggest that repeated exposure to these drugs in susceptible individuals may sensitize the brain to dopamine-the same brain chemical believed to be involved in schizophrenia. Although their long-term study identified a significant link between methamphetamine exposure and latent schizophrenia, the researchers recommend further analysis to confirm their findings, as their study is the first to investigate patterns of psychosis among methamphetamine users.