Age of Marijuana Use and Psychosis
In that other thread Fallen brought up the point that none of my studies controlled for age of exposure to marijuana, which was correct! I in turn said I would get in touch with some New Zealand people and keep him apprised, but the professor who responded to me put so much work into it I thought it would be a shame to only share it with one person.
Quick background: cable hasn't been invented yet in New Zealand, so people there have a lot of time on their hands. Three distinct groups have set up long-term (as in decades) cohort studies. They select a cohort at birth, subject them (and caregivers and somewhat ominously named "informants") to a questionnaire, then ask them the same questions again every year or so from that day to this. This form of study obviously allows for by far the most controls and sturdiest conclusions, the downside is that is enormously time-consuming but as I said, Kiwis have a LOT of time on their hands.
One of these studies is run by the University of Otago in Christchurch, and is imaginatively called the Christchurch Health & Development Study. I asked them Fallen's question, and a very nice Professor Horwood wrote back. He attached a study on ages 18-25 which I think I've already linked on this forum but here it is again. Its conclusion: "The present study suggests that: (a) the association between cannabis use and psychotic symptoms is unlikely to be due to confounding factors; and (b) the direction of causality is from cannabis use to psychotic symptoms." However, the study doesn't explicitly break down into age bins, so Dr. Horwood notes:
"Although it’s not obvious from the paper, this conclusion would not alter if we adjusted for age of onset of cannabis use."
And more important to our discussions, he goes on to state:
"We haven’t published on this issue beyond age 25 but similar associations persist between current cannabis use and the occurrence of psychotic symptoms up to age 35."
Obviously "similar" means "less" but more importantly "still present". It is worth noting two important things. The study only goes up to age 35, so the above statement does not imply that they observed marijuana to be safe beyond age 35. Second, the study and Dr. Horwood specifically refer to daily marijuana use rather than any, so it is possible that there is some lower level of marijuana consumption that while causing psychotic symptoms in adolescents will not do so for adults. Again, though, this is not implied. As everyone has insisted with comparing the numbers I cite with daily (or even higher) use for other drugs, this is not an important distinction.
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TL;DR: Marijuana use causes mental health issues up to age 35 at least. "Regular cannabis use at any age is associated with increased rates of psychotic symptoms." I'm glad that's settled! :)