Researchers are offering new ammunition to worried parents trying to dissuade their teens from smoking marijuana: Evidence is mounting that for some adolescents whose genes put them at added risk, heavy pot use could increase their chances of developing severe mental illness - psychosis or schizophrenia.
This week, the pot-psychosis link gained ground when two major medical journals reviewed the research to date and concluded that it was persuasive. In PLoS Medicine, an Australian public health policy expert wrote that genetically vulnerable teens who smoke marijuana more than once a week "appear at greater risk of psychosis," while the British medical journal BMJ noted estimates that marijuana use could contribute to about 10 percent of cases of psychosis.
The new research has little hint of Reefer Madness alarmism. Rather, a half-dozen long, careful studies published in the past several years have tried to determine whether pot-smoking is a cause rather than an effect of mental illness. And 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 pot - which would be necessary to perform a gold-standard study to definitively show 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.
But the recent research has attempted to get around these hurdles by controlling for factors such as the presence of psychosis before the use of marijuana, income, education, other drug use, and childhood traumas.
"No single study is perfect," Wayne Hall, author of the PLoS Medicine essay and a professor at the University of Queensland in Australia, said in an e-mail interview. "But the fact that so many individually imperfect studies so consistently find this relationship adds confidence to the conclusion that the relationship is causal."
The recent research points to adolescence as a particularly risky time to smoke marijuana heavily for those genetically predisposed to mental illness. Brain scientists theorize that marijuana may induce temporary changes in brain chemistry that, when reinforced over time, become permanent.
