I can't see the full study methodology without an account. That said… The challenge with running studies ethically is that you can't split your test groups into two groups and somehow induce cannabis use disorder and see if major depression, bipolar, etc results from it. Rather, you can see if the people who naturally develop the disorder also have those problems. The flaw in this reasoning is that people who develop…
> “Around 20% of young people in Denmark use cannabis but in this study fewer than 1% of the sample had been assigned a diagnosis of cannabis use disorder and these may well have been people who had a pre-existing susceptibility to depression and other psychiatric disorders.
> “Other studies have shown that cannabis use itself is not associated with increased risk of depression, so perhaps we are seeing that people with a pre-existing susceptibility to depression are more likely to be diagnosed with cannabis use disorder. This clearly does not mean that we should infer that cannabis somehow causes depression.
> “I don’t think that this study provides us with much information to help decide the extent to which cannabis use is, or is not, harmful.”
[1] https://www.sciencemediacentre.org/expert-reaction-to-study-...