We draw the line where it should always be drawn in order to maximize personal liberty. Any behavior that doesn’t cause harm to an uninvolved party should be legal.
Having said that, if you are stupid enough to use drugs, then I am patriot enough to let you.
I'm not so sure the line can be so easily drawn. At some point, the damage done to the fabric of society needs to be taken into account.
What about the families of those on drugs? Do we just let the kids go hungry because dad is a druggie? After all, it isn't OUR fault he takes drugs. If he wants the liberty to get high, then we let his family suffer as "punishment" for his addiction?
I am just not convinced that legalization of drugs is the way to go. And yes, I'm aware the same issues can be made with alcohol, which is currently legal. I'm one who was a hungry, sometimes homeless kid because of dad's drinking. Thank God mom finally gave up on him and left. At any rate, the whole "but alcohol is legal" argument doesn't get anywhere with me.