Here is a link to a study published on "The Effects of Regular Cannabis Use on Neurocognition in Adolescents and Young Adults"

A good friend of mine smoked cannabis usually multiple times daily from the age of sixteen to about the age of 21. He took breaks here in there, (but not for more than a few days or maybe a week or two.) He smoked way too much. He's had some issues with depression and mental health as well. The study above qualifies chronic use as smoking only once a week. He smoked like 5 times a day. As he got older and turned 19-20 he slowed down quite a bit. He has now stopped entirely and is making every effort to get his life together, but he seems pretty slow, detached ect.

How could this extreme usage have effected his brain? He's also been a smoker(nicotine, he has since quit though), a user of stimulants from 18-21, and a weekend user for MDMA for about a year or two. He's also experimented with psychedelics and opiates. He's actually pretty intelligent all things considered, but he is kind of slow to process or think, it's hard to tell if there has been any effect on his memory.

I don't really understand the study above, but there definitely seems to be some permanent neurocognitive deficits reported in it.

My question is what sort of cognitive deficits may he have as a result of this.