It is up to the individual to decide whether he want to use drugs or not. What he wants to do with his body, is his concern, and nobody else's.
It sounds like natural selection for me. Let human do what they want and nature will judge them. But if we could prevent our friend to use drugs, so we will do that.
And the main reason why there is so much demand for drug dark-markets is due to the fact that all the soft-drugs as well as the hard-drugs are banned by the governments. Remove the ban, and the demand will automatically decline.
(Misusing) drugs are bad, thats why government preventing this stuff for people. And removing the ban doesnt guarantee the demand will decrease. E.g porn, there are many countries that legalizing porn industry (with regulation) but the porn demand is never goes down.