Why do IV heroin users heat their heroin?

The Wikipedia page for heroin has this to say:

Why is heat required in the former case? Could a user without a heat source just patiently wait a little longer, or is heat an absolutely necessary condition for dissolution? If heat is necessary, why does it stay dissolved even after cooling?

sterilize. despite what you quoted you never find pure diacetyl morphine on the streets. it’s a powder or black tar that needs to be cooked to become liquid form. Even if the active chemical is water soluble the other impurities are not. And as i mentioned, you need to sterilize it.

Yeah, I remember a letter to an advice column in a men’s “health” muscle mag. The guy was asking about injecting steroids he’d gotten (from Mexico?) and based on the symptoms, sounded like he was getting serious deep muscle infections where he had injected.

I assume when injecting, you want to be a little more cautious than when inhaling or ingesting. You’re bypassing some very basic body defences. The substances you are playing with have probably been handled by people who were not heavily into hygiene or sterile facilities.