I think a very important part is:
> For example, I’m interested in the effects of THC versus CBD. We don't have any of that information in the UK Biobank. Most people in this study were using cannabis quite a while ago, and cannabis at that time looked very different from what’s available today. That context really matters. It’s a complex picture.
It's like having a bottle of beer vs bottle of whiskey. I don't exactly imbibe them in the same way, do I?
I know plenty of people who haven't really reduced their consumption as things have gotten stronger. Their tolerance just went up.
The researcher pretty much says "it is very complex, many things are difficult to control, so we should take all this with a grain of salt".
Your answer: "What a weird thing to say, they should just ask me; I know."
"Longitudinal and twin studies report larger declines in IQ among cannabis users than their non-using peers but it is unclear whether these findings can be attributed to cannabis use or to genetic, mental health and environmental factors."
Drinking too much coffee and getting over-caffeinated has bad effects.