There are projects where long term addicts are given pure medical heroin (Diamorphin) in a controlled environment, and they do considerably better than their control group who does not receive their drugs like that.
E.g. https://patrida.de/
E.g. https://patrida.de/
If it's not harmful it's not the same drug. Unlike diamorphine, a medical grade supply does not reduce harm; it's more like sniffing glue, inhaling poison to escape reality.