I'm someone who used cocaine heavily at a young age, from 16-19, and I'll say that depression, is often a symptom linked to the drug use. I can't say 100%, but I doubt I was clinically depressed before I started using cocaine and alcohol heavily, but after using everyday, I am sure I became so. It's usually a one thing begets another. I started off a kid with anxiety, some depression, plus typical teenage disaffection, combined with desire for money, plus drug and alcohol experimentation, and you get a teenage cocaine addict, I suspect someone like Juice was similar. There is no ONE cause of drug related depression, but one for sure cause, is the drug use itself.
You feel bad, and take drugs, causing you to feel guilty, and start the cycle all over again.
I believe opiates work slightly differently than cocaine, but similar. Cocaine fires your dopamine, like charges it really high, and slowly, you kind of scramble your dopamine reception, so you are depressed while not using cocaine, and unable to feel pleasure. All drugs and alcohol work like that.