THAT'S where you are WRONG! Drugs being illegal creates a black market which artificially inflates the price of them. If they were legal, they would be FAR less expensive. How often do you hear of people breaking into houses to steal property to sell so they can buy tobacco or alcohol? I'd wager, VERY FEW. Their illegality is 50% of the "drug problem".
As always, I give the disclaimer that I don't, and have never used illegal drugs, before you or anyone else calls me a pothead, or whatever...
I just apply common sense, and have no ideological reason against people using drugs; if they're adults and that's what they wanna do, as long as they aren't harming anyone else - and pay for their own mistakes.
That is the road to hell right there. Libertarians never recognize the harm caused because it usually happens much later, like the bullet that kills a man forty years after it hits him. Libertarians never see a proximate cause because the consequences are generally distant in time from the initiating event.
Introducing someone to drugs *IS* harming them, or is at least close enough to be regarded as reckless endangerment. Some people can try drugs and do not get addicted, others do. It is, in fact, a genetic crap shoot as to whether any given person will be swallowed up by a drug when others will reject it.