[jampekka]

Heroin was produced in huge quantities and it was a very popular drug for many ailments, including cough, asthma, insomnia and calming down children. It declined only after the 1912 International Opium Convention. In Finland it was a popular cough suppressant until the 1950s. In e.g. UK it's still used for severe pain.

Heroin (diamorphine) as a molecule isn't really substantially more problematic than e.g. oxycodone or fentanyl, which are in widespread medical use. The specific ban of heroin is more a historical contingency.