If we don’t let people have access to pure drugs under conditions that limit the risk of harm, then we should also provide them with the mental health supports they need. People will go to great lengths to kill their pain, including risking death and imprisonment. They shouldn’t be forced into choices like that.
I am not saying that’s the only difference. I agree heroin is obviously far more powerful a drug, and far more addictive. So I don’t think it should be fully legalised and sold in shops. But making it legal for addicts to use under medical supervision in special licensed places seems a sensible compromise. It would probably scale down the black market (which reduces drug pushing on new users), it would reduce immediate dangers (heroin is pretty safe when administered properly in a clean environment), and it would probably help addicts get off it and build a new life (it would remove a lot of the chaos from their lives and would expose them to drug counsellors).
I personally know 3 people that have tried heroin. It quickly has destroyed two of their lives. 0 ability to hold a job, maintain relationships, if not for family they would be homeless. My other friend was able to use it in moderation for a decade but recently going down the same path.
The idea that heroin is less harmful then alcohol is absolutely ludicrous. The reason it has done less economic harm is because it's less accessable, extremely taboo, illegal, etc.