That is, we should assume they cause long term impairment without the need for any kind of study?
If a study comes back and says, with a high confidence, that isn't the case for this or that, great, but I think it's a safe assumption for most chemicals.
Edit: Just to be clear, I'm saying it's not enough to show something is bad, as pretty much everything is bad. It has to be bad enough to matter.
I think we are mostly in agreement - that any substance entering the body will have some effect.
The question is what effect, how much of an effect, and wether it is permanent/long lasting. Questions which require a study to answer.
Unless we're able to find twins that live largely the same lives absent the one factor, we are really only capable of sussing out the especially harmful things with long term lifestyle studies. If cannabis is as bad as sugar, then we have 40 years of back and forth to look forward to, and the answer will ultimately be: A little bit is bad, but not bad enough to matter more than how many steps you took today, or how sad or angry you were, or how late you ate dinner.
Alcohol is clearly capable of long term impairment, the debate seems to be about dose response. Nicotine is more questionable, but clearly leaves long term changes in the mind. People abstaining after long term nicotine usage will spend the rest of their lives experiencing cravings. Hell, it seems plausible long term consumption of food types has long term cognitive effects.
There is definitely some impairment long term. I think we can assume that even without this huge study. I don't think knowing that matters unless we have some scale that tells us how dramatic that impairment is.
If I compare someone who smoked all the time, but also had a rich and engaging social life with a lifelong abstainer who live a mostly isolated existence, who is going to be more cognitively capable?
There are so many inputs that feed into who we are and what we are capable of that, in all likelihood, cannabis use is likely only incidental to much larger factors like isolation, media preferences, home life, etc.
As an anecdote, the worse thing I think I've ever done for my brain, the thing that I immediately felt dumber after doing, was reading "Shadow Wolves" (Steven Segal's book). I had trouble communicating thoughts effectively for weeks after finishing it. It was like it had sucked vocabulary out of my brain. Maybe that's how cannabis leads to long term impairment. It makes you read "Shadow Wolves".
Edit: Since people are voting this, I want to add an extra warning that I was not joking about "Shadow Wolves". If you read it, you will feel dumber, and it will take time to heal the damage done. Seriously.