The most recent study to date is a case in point. The researchers examined 901 people aged 18-64 who experienced first-episode psychosis at a mental health facility, compared with a control group of 1200 people in 11 cities around the world. While it still does not prove causation, it did find that 1) daily use of cannabis in general, 2) initiating use before age 15, and 3) use of high-potency cannabis (i.e. above 10% THC), especially when used daily, were correlated with a higher risk of having such psychotic episodes.
And while the three cities with greatest availability of high-potency weed (London, Paris, Amsterdam) had higher rates of psychosis, though this was cross-sectional and cannot establish temporality, a rather crucial criterion of causation. And interestingly, psychosis rates were higher in London and Paris (where weed is illegal) than in Amsterdam (where it is quasi-legal), so the legal status of cannabis did not really appear to be an issue. Moreover, it is not clear which is the proverbial chicken and which is the egg.
Paul Armentano of NORML wrote an excellent and in-depth article (with links to other studies) that does a great job debunking this latest round of Reefer Madness 2.0, and is certainly worth a read. A shorter version can be found here as well.
Additionally, we at Twenty-One Debunked have previously reported on the emerging research suggesting that nicotine may be the real dark horse here in the etiology of psychosis and particularly schizophrenia. And famously in Europe, particularly in those three aforementioned cities, cannabis is usually mixed with tobacco, especially when it is high-potency and users may wish to stretch it out by giving it some bulk. Though clearly a far less toxic and nicotine-free way to stretch it out is mixing with catnip or even parsley, or just do what most American tokers do: smoke (or vape) the weed straight up without mixing, but simply take fewer hits, use bowls/bongs instead of joints, and/or seek out lower potency strains. And in Australia, not mixing with tobacco is about as common as mixing.
And as we have noted time and again, the rates of cannabis use have increased dramatically since 1960 or so in nearly every country, while the best evidence suggests that psychosis rates have generally NOT increased since then. Thus, any causal link with psychotic disorders that would not otherwise have occurred would most likely be either weak, rare, or both, and thus likely limited to a very, very tiny percentage of the population who are unusually vulnerable and/or ultra-heavy users. And while some already psychotic users may find exacerbations of their psychosis (or its effects may make a latent or subtle psychotic disorder more noticeable to themselves or others), others may use the herb to self-medicate as well, and everything in between.
Overall, this and other studies seem to be more smoke than fire, and even if causal they do not suggest that the hyperbolic "no safe level of exposure" theory is correct. Rather, the biggest takeaway from this study is, if you do use cannabis, don't overdo it, particularly with the high-potency stuff. In other words, moderation is the key. And while age at first use was one of the lesser factors, delaying use until at least age 15 or older is likely prudent--not just for this reason but for other reasons as well. And these studies do not really militate against legalization, but rather should be seen as encouraging legalization and proper regulation and taxation of cannabis based on THC and CBD levels. Nor do these studies support setting the age limit any higher than 18 either (which of course will only encourage the black market to persist). So we need to see the forest for the trees.
And thus we will finally have Reefer Sanity in this country for once.