I'm flattered that
a table that I added to Wikipedia has been reproduced in this thread.
Based upon the most common definition of a drug (i.e., "any substance (other than food that provides nutritional support) that, when inhaled, injected, smoked, consumed, absorbed via a patch on the skin, or dissolved under the tongue causes a physiological change in the body),
sugar isn't a drug because it provides nutritional support. However, the physiological effects of sugar on the body are, to some extent, analogous to addictive drugs. Moreover, like psychoactive drugs, the physiological function of sugar (specifically,
glucose, since this is the form of sugar that the human body utilizes in cellular metabolism and stores within various organs/cells in the form of
glycogen) in the brain directly affects cognitive functions and perceptual cognition.
In relation to addictive drugs: the consumption of sugar compounds like sucrose (or just palatable food in general) by a hungry animal induces dopamine release in the ventral striatum, including the nucleus accumbens, via the mesolimbic pathway. It also activates the
hedonic hotspots (i.e., the brain's "pleasure centers") in the nucleus accumbens shell and ventral pallidum. In other words, sugar consumption directly activates the epicenters of the
reward system in the human/mammalian brain. The primary reason for this is evolutionary: consuming food promotes survival of an organism and more generally the species as a whole, therefore the behavior is naturally rewarding (i.e., it acts as a primary reinforcer, as indicated in the table above).
To the extent that "sugar" (glucose) is used in cellular metabolism by brain cells (neurons and neuroglia in particular), it's a psychoactive substance. This is particularly apparent in cases of
hypoglycemia, which involves CNS-mediated symptoms, and pronounced states of
glycogen depletion in the periphery (i.e., depletion of glycogen stores in the liver and skeletal muscles) and central nervous system (glia and cerebral blood plasma) which sometimes occurs in marathon or ultramarathon runners (more commonly in ultramarathon runners); in the latter case,
hallucinations sometimes occur.