If the results of whatever illegal substance you put into your system endanger the lives of others, then there is your line in the sand. If you eat 20 Big Macs a day and still want taxpayer funded “free” healthcare, then the rest of us have a problem.
So, if the said person were to consume the aforementioned ‘illegal substance’ in the privacy of his or her home, without endangering anyone else while the substance runs its course, would the actions be “legal”?
As long as you have an external entity deciding for you what you get to put in your mouth and what you don’t, fundamentally, there is no true freedom. There is no argument which can justify tobacco and alcohol being legal, by applying the premise of your argument.
Erase all laws that deal with drugs and say with a straight face we do not already have laws in the books to handle (sober or intoxicated) one citizen infringing on the liberties of another citizen.
DUI and Drug laws are nothing more than feel good legislation with the primary result of the Government exerting more power over the individual and filling its coffers with "fines" that make the MADD crowd giddy with excitement (same thing with the drug warriors).