I suppose my first exposure to this question came at a young age, when I read in the Charlie Brown Encyclopedia, Volume One:The Human Body about what a drug was. Althought this topic may have been a little risque for the Charlie Brown Encyclopedia, they handled it with remarkable ease, describing a drug as something that changes the way the body works.

Which is a good enough answer, although translating this axiom into something that explains the myriad rules and regulations that our government has set up to let us know which chemicals are drugs and which ones are foods is a rather hard job. Anyone reading this can insert their favorite or least favorite example of something that is considered a food (chile peppers, melatonin, hormone laced swine, partially fermented orange juice, Hershey's Chocolate) into the equation and try to figure out how, and why, these things are considered foods and not drugs.

There is a conceptual way of understanding the difference, or perhaps why people think there is a difference. It will involve some softcore math because I think that is the clearest way to explain it.

One of the easiest ways to understand it is by the analogy of a cup, or a browser window.

  • Suppose you have a cup, and it has some filthy, foul liquid in it. You pour the liquid out and fill it up with some nice, not very fermented orange juice. Then, after you are done, you use it to wash your teeth with, and spit in it. You empty it out.
    The next day though, while cleaning your stove, you put some lye in your cup. You can no longer drink out of this cup. The essential nature of your cup has now changed. It can not be altered back to what it once was.
    Conceptually, the various liquids in the cup were like food. They didn't change anything about the cup, and after they were through, the cup could easily be changed back to its starting point.
  • Suppose you have a browser window. It is some form of righteous, politically correct browser like Mozilla or Lynx. You are merrily surfing around the web, looking at righteous sites like theonion and slashdot. Then, just for a hoot, you cruise over and visit msn or aol. Visiting these sites is not the same thing as installing internet explorer or the aol "browser" on your system. (And I have had to explain that many times to people in tech support).
    So, the analogy might run that a normal functioning "undrugged" computer running Mozilla can eat health food (slashdot) or junk food (msn), but to actually install a different browser on the computer is the equivalent of using drugs.

That being said for you verbal learners, I think the mathematical notation explains it the best. Imagine that the body has a function. Let's give the body this function:
Now, we can feed any number we want into this equation, and each time we will have a different output. Imagine the different numbers are food. The different foods give us different outputs, but the function itself doesn't change. And because the function doesn't change, we can always go back to getting a different output.
Now, this is the function of your body on drugs:
Or something like that. The function itself has changed. That is, when different "normal" inputs are put into the body, a totally different output is gotten. There is now no way to go back to the old ways by putting in a different input.

That conceptual difference being explained, and it is a real one, I have some critiques to say about this theory in practice. For one thing, the human body doesn't have a "function" in any means of the word, and anyone attempting to explain this function in terms of variables would need thousands of variables in many many dimensions. And trying to turn a multi-qualified thing such as the human life process into a single line that has either good or bad sides only works in one places, RPGs (and at least they also give you MP).

The human body is not a pot or a cup or some other form of receptacle. It does not passively hold and process some substances while letting others malevolently take it over and turn it to evil.

If we could define a single life process that the body was doing, and could identify things that were either fodder for that reaction or could actually change that process itself, then we could define a drug or a food for that process. By extension and using our sense, we could define something that blocks a process such as the Krebs Cycle as a dangerous drug.

In addition to the conceptual shallowness of the food/drug dichotomy, I think many of us here could point out certain cultural biases that persist in existing, despite shaky evidence. For example, m+arijuana is considered a drug legally while peppers are considered a food. However, in Montana, do hikers carry large amounts of readily inhalable cannabis with them? (well, actually they do), but not to ward off charging grizzlies.