LOL!
The libertarian conceit assumes everyone will behave rationally in the absence of external controls and laws. That includes junkies. There are no addictions is a libertarian free state--just responsible casual indulgences. It is the law--not the substance consumed--that destroy bodies and souls.
But you are right. A junkie will scrounge a used syringe and needle from a dumpster or gutter and use them before he will walk into a shop fifty feet away where syringes are sold without a prescription and pay fifty cents for them unused. Having used them, he will drop them in the dirt and the next junkie will spy them and use them. And so on, and so on.