> A similar finding is that kids who smoke marijuana are likely to experience serious mental health issues later on -- that's indisputable. The problem is the same as in this case -- people assume a cause-effect relationship that isn't supported by the evidence.
This is one of those topics where people will set an impossibly high standard for evidence and then use that to reject everything.
We do actually know that heavy marijuana use is associated with increases of mental health conditions. We know that discontinuing heavy use can result in improvements in depression scores over time (following the withdrawal periods which, yes, exists despite decades of people claiming it doesn’t). We know that the increased availability of high concentration THC products has made it worse.
Yet if you want to set your threshold for evidence so high that nothing short of a large scale RCT will answer the question you can ignore all of this, because such an RCT will never be produced for obvious ethical reasons.
This pattern is common in topics where people simple don’t want to believe a casual relationship might exist. I knew a guy who started believing that there isn’t any strong evidence that flossing or even twice-daily brushing were important because he didn’t think the science proved causation that flossing and brushing improved dental health. He believed it was just correlation and people who were healthier in general were also flossing and brushing, coincidentally. His breath was terrible and he later abandoned those ideas when the dental problems undeniably arrived.