Did the people you notice becoming less intelligent ever recover? I'm genuinely interested. My biggest regret in life is early years drug use, smoked my first joint at 13. Mdma 18. Cocaine late tewnties. I personally think marijuana might be worse than mdma but not by much. And cocaine is really bad for cardio vascular system, probably physically worst of all of them that I tried.

I think both mdma and marijuana cause anxiety and they mess with short term memory.

There doesn't seem to be a good answer to protecting kids from drugs. Heavily regulated legalisation might help or it might normalise drug use.

As an aside I personally think alcohol in very moderate use isn't really as harmful as other drugs. And is probably a net benefit for many. Even moderate use of illegal drugs seems to have bad affects on people.

Edit: added my thoughts on alcohol and something on cocaine use.