I’ve been hearing this all my life, and I’m sure most of you have too in one form or another. But in my own limited understanding of biochemistry, I can think of no apparent reason why this would be true. Yet I also know that I sometimes crave sugar, particularly at night. I’ll temporarily suspend the hypoglycemia hypothesis for the time being, as I’ve been led to believe that the concept of hypoglycemia has been largely discounted by the scientific and medical community. As I understand it, true reactive hypoglycemia is a relatively rare phenomenon, and is indicative of a severely deranged endocrine system. That being said, does any metabolite of sucrose cross the blood-brain barrier and latch into the brain’s reward circuitry? Is that why people “need” a sugar fix? If so, what is this metabolite? Or is it just that the brain is running low on glucose and says “look, Bubba—I’m running low on available glucose here, and I don’t feel like waiting around for a complex carbohydrate to be digested”. “Can you get me some sugar ASAP”?
From dictionary.com , one definition is :"any article, other than food, intended to affect the structure or any function of the body of humans or other animals. " Since sugar, a carbohydrate, is technically a food, it cannot be a drug.
Any lack of nutrition will cause the body to crave it to replace it. For example, lack of water (not strictly nutritious) will cause a strong desire to drink. Hunger as well does this. Many times, the body will crave exactly the type of food it needs, probably through nurture-training.
Having been on the up-and-down swings of blood-glucose levels (diabetes), I can attest to behavior and attitude changes. But through discipline, I reject my craving for sugars and carbohydrates, and reach for foods that are higher in protein and/or low in sugars. (Fiber too, is a plus.)
(OK, I admit I just ate two fajita pockets. But I’m gonna eat a bunch of carrot sticks and spelt crackers to compensate. 
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It depends on the definition of “drug”. I read a book a while back which claimed that sugar was indeed a drug. I think the book was called On Drugs, but I’m not sure. If I remember correctly the author argued that any substance which alters a person’s mood could be considered a drug.
Do you consider anything you “crave” to be a drug. Yeah, metaphorically, anything can be a drug . . . sex, music, religion, etc. But literally? Not using the normal definition of “drug.”
Is water a drug? I ask because I guarantee that you’ll crave it if it’s withheld from you.
Less facetiously, although the pathways for basic glucose metabolism are very, very well understood, there’s still considerable uncertianty regarding how glucose and/or its metabolites act as signals intracellularly. The example with which I’m most familiar has to do with the insulin-producing beta cells of the pancreas (aka islet cells). It is still poorly understood how glucose entering these cells stimulates them to produce and secrete insulin. Is it the glucose itself? A product of glycolysis? Maybe energy (ATP) generated via glycolysis? Call back in a few years. (BTW, if you’re interested, and have a maschistic bent, look here)
Ok, GQ answered I can now go off beat a little.
I picked up a collection of old stories from 2000AD and came across a story relating how sugar was one of a list of banned substances in the future, after it was decided that it was causing too many medical problems. Just around the same time the UK started its anti-salt campaign, is that life imitating art?
Vision correction, in the form of glasses or contact lenses is a vicious drug. For many people, once you’ve experienced it you are hooked for life.
And oxygen – just one breath leads to a permanent addiction that invariably ends in death!
(I think that’s from a longer Isaac Asimov quote.)
Is alcohol a carbohydrate?