Examples without names, personal friends of mine:
1. Alcoholism, watched this man crawl from prodigal son to non-productive leech.
2. Weed. I have no problems with recreational weed on some occasions, and have enjoyed it myself, this person has their mind constantly fried. Kid went from a 16 year old CS stunner to a 22 year old, reaction shot, deep-fried potato. Maybe not the 'smartest,' but he had major future in his field.
3. Xanax. Friend of mine, beautiful content producer, amazing aspirations. While being majorly depressed he moved from SSRIs which weren't helping him, to taking Xanax he'd buy illegally. Slowly got out of hand while abandoning his aspirations, classes, and more. He's no longer working. He committed suicide a few weeks ago.
4. Xanax, again. Dear friend of mine. Closest person I've ever known. Instead of long-term medications to deal with her issues, she turns to Xanax as a short-term fix. The immense pain caused by whatever is bothering her at the time turns her to Xanax, which in turn normally ends in her becoming unproductive for the rest of the day and sleeping for 12-16 hours. She's never been able to find anything else that helps, and in turn without Xanax she cannot handle problems well. Breakdowns, endless crying, hyper-focuses on topics. I fear things will not change for the better, although I still try.
Mental illness / mental differences are other slow disasters that my friends have much experience with. Many of us have one of many things, most commonly autism or ASPD.
There are others, too many to name that I feel have been lost for a reason or another. Sad shit.