My knowledge is mostly from living in an area where most addicts were heroin or other downer drugs. While there were a few who had problems with coke and speed most of the junkies I knew were on heroin. And when a strong package is released to the street people start dropping. There are even signs in prisons telling people to be careful when released because the stuff on the street is stronger than in prison.

If you look at who generally dies from drug overdoses it's largely opiate-based drug users. I once listened to two junkies who hadn't seen each other for quite a while talking and letting each other know about who died. They were mostly talking about overdoses, the conversation went on for about 30 minutes non-stop with different names non-stop. None of the cokeheads, eckyheads (MDMA), or speed freaks I knew ever had conversations like that.