I've witnessed something similar, and I later discovered the person was developing schizophrenia. It seemed like the drugs were setting him on a bad path, but I believe he took them to self medicate in retrospect. I suspect that's fairly common now.

He's still very intelligent, but all of his mental resources are wasted on fighting his illness so to speak. To some it might seem like he ruined his brain with drugs but I sincerely doubt the drugs did much damage at all.