The new review suggests that even infrequent use could raise the small but real risk of this serious mental illness by 40 per cent.
The scientists found a more disturbing outlook for "heavy users" of pot, those who used it daily or weekly: Their risk for psychosis jumped to a range of 50 per cent to 200 per cent.
"We've reached the end of the road with these kinds of studies," said Dr. Robin Murray of King's College, who had no role in the Lancet study. "Experts are now agreed on the connection between cannabis and psychoses. What we need now is for 14-year-olds to know it."
There was one curious admission by the researchers, akin to the "gateway drug" argument:
There could be something else about marijuana users, "like their tendency to use other drugs or certain personality traits, that could be causing the psychoses," Zammit said.
Nothing new, in that other studies have shown that marijuana users are statistically more likely to use other drugs. I've never bought into the argument that marijuana use translates into a path to "harder" drugs, because you could just as easily conclude that those that drink are more like to try pot, than non-drinkers. Is booze a gateway drug?
My point, as it relates to this study. Is it not inherently problematic to conclude that marijuana is the cause of increased psychosis, when your sample group is contaminated? The researchers obviously found marijuana users had a higher "tendency" to be users of other drugs, which undercuts an isolation of marijuana as root cause. You have a cocktail of potential culprits, which should serve as a red flag, in drawing any definitive conclusions. If the study limited itself to people that had only used marijuana, you still have the obvious environmental problems, but the case is far more persuasive. It is akin to studying the effects of steroids, with some of the patients also taking growth hormones. A small "skunky" smell with the sample group, which might detract from "we've reached the end of the road with these kinds of studies" proclamations.