A rather odd question, really. What with the medical issues being taken care of (I was gonna go with penicillin) and the resolution of other issues, such as the definition of a drug ("a substance that has a physiological effect on the human body") being broad enough to cover, well... everything, really, from food to water to breathable air (and if you don't think those have physiological effects, go without them for slightly less than the lethal time period and add them back for some startling results)... well, the answer seems to be determined almost entirely by the holes in the question.
Caffeine, for example, seems to be the most popular answer... but how many people have said that because they actually, to an extent, need it for medical purposes? If your boss wakes you up at three in the morning and says you have to start an eight hour guard shift right now (which happens, for those of us in the lower ranks of various militaries) is that cup of coffee you grab on your way a luxury, or is it a very important medicinal remedy for the medical condition of falling asleep on the job? I mean, sure, you could try to fight the encroaching sleep the old fashioned way, but you could also try to get over many severe infections on your own, with similar chances for success and similar levels of stupidity. Where do you draw that line?
Just strikes me as odd, I suppose. Seeing as I don't smoke, don't drink, and don't use any sort of illegal substance (let alone more than one, which would make this an actual choice) I suppose this question just isn't for me.