Uneducated and influenceable people currently consume heroin, not realizing their act.
I would be tempted to think the fact drugs are rare and difficult to obtain the first times decrease the odds of somebody becoming an addict. Pure speculation but there is some logic still. Say one night I'm crazy, I'm not going to find cocaine right away become hey, tonight is the night. I understand that it's not a proof that it's effective. Just that it might be.
Heroin is comparable in risk profile to morphine
Doctors prescribe morphine, most likely when not using it is worse than using it, which doesn't mean the risk are low. It's just the best of the worse solutions. I understand that when you lost a leg, you can take some hard drug, because anyway, if you don't you're done. I might be wrong again, just telling you the arguments of the common here.