Research & Medicine

Few arguments turn up in the cannabis debate as reliably as this one: “Cannabis causes psychosis, so obviously the stuff is harmful.” The sentence sounds final. It isn’t. The research on cannabis and psychosis paints a far more precise picture, one that neither plays down the risk nor lumps every kind of use together. We went through the current evidence and worked out what it actually says. Cannabis and psychosis: a link nobody seriously disputes Let’s start with what holds up. There is a connection between cannabis use and psychosis, and it is well documented. People who use regularly have, on average, a higher risk of developing a psychotic disorder or schizophrenia than people who never use. That is not a fringe position; it runs through dozens of studies and reviews. But the slogan “cannabis causes psychosis” glosses over the crucial point: how much, how potent and who account for almost the whole difference. That is where it gets interesting, and that is exactly where the blanket harm argument falls apart. How high the risk actually is Before talking about risk factors, it pays to look at the baseline, because without it, talk of “five times the risk” means nothing. The lifetime risk of developing schizophrenia in the general population sits at roughly 0.6 to 1%. Counting all psychotic disorders together (which includes delusional, schizoaffective and substance-induced forms) it rises to around 3%. Isolated, temporary psychosis-like experiences, without any illness being present, are something an estimated 5 to 7% of people have had at some point. That single percentage point is the anchor for everything that follows. When a study reports a “doubled” or “fivefold” risk, it almost always means relative risk, measured against this baseline. A fivefold risk sounds like 50%, but starting from 1% it works out to roughly 5%. Keeping that difference between relative and absolute risk in mind gets you halfway to an honest debate. The risk map: where cannabis actually sits With that baseline in hand, you can put the whole list of known risk factors in order, and you see immediately that cannabis is not at the top of it. The overview below pulls together what meta-analyses report in terms of relative risk increase, along with a rough absolute reference point. Factor Risk (relative) Absolute lifetime risk (approx.) General population (baseline) n/a ~1% (schizophrenia), ~3% (all psychoses) Identical twin affected n/a ~40–50% Both parents affected n/a ~40% Parent or sibling affected ~6x ~6–10% Racism and discrimination ~4x ~4% Childhood trauma (multiple, 5+ adversities) ~3x (up to ~6x) ~3–6% Urban birth or urban upbringing ~2–3x ~2–3% Cannabis: daily and high-potency ~5x ~3–5% Cannabis: frequent use (weekly or more) ~1.3–2x ~2% Migration or minority status ~2x ~2% The absolute figures are rough reference points based on a baseline risk of around 1%. The factors do not simply add up; some of them interact or overlap. Relative risks for discrimination and childhood trauma follow the 2021 Frontiers in Psychiatry umbrella review and Varese et al. (2012). Two things stand out. First, predisposition dominates everything else. Heritability estimates run from 60 to 80%, with a Danish twin cohort landing at around 79%, and an affected identical twin lifts the risk into an order of magnitude that no single environmental condition reaches (Hilker et al., 2018). Second, cannabis lands in the middle of the environmental factors: the daily high-potency extreme case is on a par with discrimination and childhood trauma, while ordinary use sits below that. Then there is the population-level view, which flips the public weighting. For society, what counts is not only how strongly a factor acts but how widespread it is. Adverse childhood experiences are common, which is why an estimated third of all psychosis cases can be attributed to them statistically, more than to any other single factor (Varese et al., 2012). That is precisely why social adversity, trauma and discrimination often sit at the top of the list in prevention research, even though genetics carries the most weight in any individual case. On this map cannabis is one factor among several: real, moderate in size, and unlike genes, background or childhood, one of the few you can influence directly through your own behaviour. Where the risk starts to rise In 2022, a Canadian meta-analysis pulled together the evidence on how the frequency of use affects psychosis risk, across ten studies covering around 7,400 people (Robinson et al., 2022). The result is remarkably clear and rarely quoted, because it is hard to turn into a scandal. People who use only occasionally, yearly or monthly, show no statistically significant increase in risk. Only from weekly use onwards does it climb measurably, by around a third. At daily use it then sits at roughly double that of non-users. So there is a threshold below which the data do not show an elevated risk. That is the first nuance the slogan loses: the relevant question is not “use, yes or no” but “how often”. Potency is the real crux The second nuance matters at least as much, and it explains why the debate looks different today than it did thirty years ago. That THC potency has risen steadily is well documented (Hoch et al., 2024). The figures usually cited: in the 1980s, THC content in cannabis was typically 2 to 4%, while today flower in legal markets often reaches 20% and more, and concentrates 80 to 90%. It is no longer the same substance. A British longitudinal study tackled exactly this question (Hines et al., 2024). It followed adolescents over years and asked whether high-potency cannabis in adolescence is linked to psychotic experiences later on. The result: those who used high-potency cannabis as teenagers had roughly double the risk of new-onset psychotic experiences. For “any” cannabis use, by contrast, the association was weak and statistically uncertain. That is the decisive finding. The effect hangs on potency, not on use as such. An even more recent analysis from the European research network EU-GEI marks the upper end of