This post does not even begin to address the negative consequences of drug use. On the other hand, a position in favor of prohibition does not eliminate the negative outcomes you have described, either.
All rights come with corresponding duties. If an individual has a legal right to purchase and consume a drug of choice, he also has an obligation to avoid doing physical or financial harm to others, including taxpayers, in the process.
How can this responsibility best be imposed upon the person who claims the liberty?
That is one of the most concise presentations of this issue's double-bind I've read.