Harm reduction practices (no redosing/high dosages, sleeping on time, proper nutrition, body temperature mgmt.) should really keep any damage at bay.
I'd say meth is one of the safer drugs if one has self-control, which is thrown out the window once you're high.
Benzos/alcohol - WDs can kill, high addiction potential that creeps up on oneself
krokodil - you literally rot from the inside out
Krokodil isn't very common and it's not the actual drug part that causes the harm. It's all the adulterants. I point this out cause to not have it said is misleading.
If you ask me, there is no clear answer to this question. I mean we can objectively lay out all the bad shit about each drug and subjectively evaluate it all, but it isn't a very accurate picture. Because in practice what drug is the worst drug varies from person to person.
Pretty much anyone who knows the reality would agree that methamphetamine is physiologically more harmful than heroin. And it is more harmful in reality for some people.
But others, like me, it's heroin that's the worst. Because heroin is addictive to me. Meth isn't. So I might use meth sometimes, but it'll likely never do me the kind of harm heroin has played a part in with my life. But for someone else it could be the other way around.
Some people, alcohol is the worst drug for them to use, but it isn't for me. Cause I can control my use of alcohol. Some people can't. Some people try heroin and don't enjoy it and won't have it destroy their life. But for me, it lead me to losing virtually everything of value I had in my life.
So yeah, I don't think this question has a simple answer. We could talk about which drug is likely the most physiologically harmful. But what would be the point? Apart from a purely acedemic question as something to talk about, it doesn't matter which is the most harmful in that sense, cause it says nothing as to which drug will actually be the most harmful to any individual person. The answer is again, misleading.