Basically, it’s harmless.
Addictive yes, but worst side effect is constipation. (Visions of heroin addicts storming shops stocking senna and liquid paraffin.)
Black market heroin becomes poisonous and dangerous because unscrupulous dealers cut it with all kinds of pollutants. — Nick Davies
The available evidence indicates that heroin, when provided in pure form, is a relatively safe drug. Hence it is primarily the illegal nature of the drug, rather than its pharmacological properties, which leads to the health and social problems associated with its use. — Ostini, Bammer, Dance and Goodwin. 1993. ‘The Ethics of Experimental Heroin Maintenance’, Journal of Medical Ethics
In other words, we f**k up heroin addicts; the drug doesn’t.
In the late 1960s, there were a few hundred heroin addicts in the UK, mostly living in central London. They got their heroin not from dealers — but on prescription from psychiatrists.
Then, following an outburst of media hysteria and pressure from the US, the authorities forced the doctors to restrict the supply. A black market was created. Addicts began turning to crime. New supply chains to the East opened up. Thatcher completed the disaster, shutting down legal heroin altogether in favour of methadone. Twenty years after the clampdown had begun, that stable population of a few hundred users had expanded to 300,000 criminals and basket cases, linked to around 70% of all break-ins, shoplifting and robberies in the UK.