We sent our (then) 19 yr old son to rehab for what we thought was an out of control marijuana addiction. Turns out he'd been doing any/every drug he could get his hands on, including huffing and eating fentanyl patches. As I've heard often, and since come to believe, whatever you think they're doing, multiply that by at least a factor of 5.
He managed to keep his last job for 18 months. He lost it eventually and can't find another one to save his life. Not hard to figure out why -- one look at him and you can tell he's an addict: dirty, wrinkled clothes, circles under his eyes and about 30# underweight.
So yeah, he went downhill very quickly in the last 2.5 years. VERY quickly.