Evidence is mounting that for some adolescents whose genes put them at added risk, heavy marijuana use could increase the chances of developing severe mental illness: psychosis or schizophrenia, reports the Boston Globe. Two major medical journals reviewed the research to date and concluded that it was persuasive. In PLOS Medicine, an Australian public health policy specialist said genetically vulnerable teens who smoke marijuana more than once a week ”appear at greater risk of psychosis.” The British medical journal BMJ cited estimates that marijuana use could contribute to about 10 percent of cases of psychosis.
The new research has little hint of ”Reefer Madness” alarmism. A half-dozen long, careful studies have tried to determine whether marijuana-smoking is a cause rather than an effect of mental illness. Groundbreaking research has begun to try to pinpoint which genes and brain chemicals could do the damage. The conclusions remain controversial, in part because it would be unethical to randomly assign teens to smoke or not smoke marijuana — which would be necessary to show definitively that adolescent marijuana use causes mental illness. It could be the other way around, or some other factor could put teens at risk of both. Recent research attempted to get around these hurdles by controlling for factors such as the presence of psychosis before the use of marijuana, family income, education, other drug use, and childhood traumas.