That's fine, I don't use any of those so I'm not an expert. There were 3 heroin ODs in my rural county a month ago, which is new. Previously the non-alcohol-substance-related deaths were all meth. Also visually I can't really separate meth users from heroine users. They have the same vacant stare and horrible skin. I guess you could say their teeth are fucked up in different ways. The coke users I've known have been completely functional, with no obvious drug-related health issues.

Regardless, it seems a bit reductive to theorize about a path-dependent phenomenon like addiction solely from the perspective of blood chemistry. Saying "substitute" implies that meth-heads are switching to heroin, which I already denied to be the case. They aren't switching to heroin, but their younger siblings, cousins, aunts, and uncles are using heroin rather than meth.