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. 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.
Going by your logic is it okay to let someone commit suicide because it is his life and body? If we know for sure that something is harmful for individual and for society,we should take all the necessary steps to prevent it from happening.Drug causes several physical and psychological problems and should be avoided.Also it is absurd to say that drugs demand is high because of ban.Nobody buys drugs to violate ban,they buy because they are addicted to hit.Removing ban will only cut down the drug prices it will have no affect on its demand