Many marijuana prohibitionists insist one reason to keep marijuana illegal is that it causes mental illnesses, despite many scientific studies that say otherwise. A new study conducted through the Harvard Medical School adds to that body of research and shows that marijuana use is unlikely to be a cause of schizophrenia. The researchers studied four groups: people with no history of psychosis and no history of marijuana use, people with no history of psychosis who were heavy marijuana users as adolescents, people with no history of drug use who have been diagnosed with schizophrenia, and those with a history of heavy marijuana use during adolescence prior to the onset of schizophrenia. This is the first family study that has included control groups with both healthy marijuana users and those who are healthy and have never used marijuana. The researchers collected information on family medical history, particularly if any first, second, or third degree relatives had any psychiatric illness.
The results of the study show that marijuana is not a likely cause of schizophrenia, while the researchers found that it is a family history of mental illness that is the real indicator and cause of illness.
The researchers concluded that the results of the current study, “both when analyzed using morbid risk and family frequency calculations, suggest that having an increased familial risk for schizophrenia is the underlying basis for schizophrenia in these samples — not the cannabis use.