"An urge is just another name for an addiction/craving. The body might very well be telling your brain that it's missing something, and expecting you to replace it. If it falls in the realm of an addiction, then all bets are off. Noone knows conclusively what the mechanics of addiction are, much less how to stop it. If it's a craving related to the condition of the sufferer's body, then that is something we can more readily sort out and investigate."
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"Addiction" is VERY well understood, and is a function of our sensory neurology. Specifically, the senses of Smell and Taste, which operate based upon the molecular "lock and key" neuroreceptors located in the mouth, nose, and key input/recognition areas of the brain (for the purposes of this topic). Since all substances have specific molecular configurations at any point in detection and metabolism, they will only interact with neuroreceptors having the corresponding configurations. Also, neuroreceptors are often capable of accepting more than a single substance, if the molecular configuration is nearly identical (such as certain opiates and their synthesized "safe" counterparts used in treatments, including Methedone).
A key point should be recognized here. While molecular configurations of neuroreceptors are "object" (identical for each specific type of receptor from one person to the next), the numbers of each type of receptor are not. Neither are the neural pathway associations formed in detecting and experiencing them. Some are "hardwired" genetically, if you will. But the majority are spontaneously created as a result of the over-all influences at the time of the experience. So each of us possesses common yet unique abilities for forming neural association due to substance detection and metabolism. We each come "pre-packaged" with unique genetic predispositions in terms of the relative numbers of any given type of receptor, and we also have distinct and varying experiences during the association forming process.
This means that one person with a genetically low number of receptors (we'll call them Type#A), that experiences some new scent or taste in a relatively quiet environment without strong emotional influences, will have a predisposition to forming a relatively weak association as well as perceiving it as a mundane experience. By contrast however, the person with a genetically high number of receptors (still Type#A), that experiences some new scent or taste in an excited environment with very strong emotional influences, will have a predisposition to forming a very powerful association as well as perceiving it as an intense experience. Which is the result of the stronger electrical signal generated by the additional receptors all firing at the interaction with the molecular configuration of whatever substance they interact with.
Obviously, particularly in the latter case, this merely demonstrates the mechanism of the experience and neural pathway association formation. It doesn't determine the relative sense of Pleasure vs. Pain (or Good vs. Bad, if you will) that becomes more significant in more intense experiences. The point being that addiction always hinges upon 2 principles. First, that there is a strong state involved when the addictive association is formed. Secondly, that the momentary new state perceived during the formation of the new pathway is stronger and more pleasurable than the pre-existent state and association. This is why it is so common for genuinely sincere individuals to "fall off the wagon". It is not exactly a lack of Will Power, so much as an internal neurological default to the stronger state association. Until the addictive pathway is collapsed and superceeded by a newer stronger one, there will always be that default predisposition toward the addiction.
The mechanism is not a simplistic Causal one either in that "It's physical, and there's nothing I can do about!", or "It's all in my head, and I'll just Will it away!". The "Will" approach occaisionally APPEARS to succeed, but only because at some point a new association has been created to override the addictive one. Which is never truly accomplished by pure Will alone, but rather through comprehension and understanding of the nature of the process and/or the perception of real need and desire for change. The term used in desribing this psychology/physiology cyclic interaction process is known as a Cybernetic Loop.
Now it strikes me that there may be a key to "blood drinking" here. Recall the mention of genetic predisposition toward the relative numbers of a given type of receptor? If certain individuals have anomalously high numbers of a given receptor (we'll call it the Type#B receptor), and that receptor is configured to interact with some key component in the bloodstream, then it logically follows that such individuals would be genetically predisposed toward a stronger experience and association to the taste of blood. That alone might be enough to create a subconscious memory file of the experience. The body filing away that it noticed a strong response to this stimulus which was the result of the stronger electrical signals generated by the additional receptors all firing at the interaction with the molecular configuration of whatever component in the bloodstream they interact with.
What comes most to mind is the blood's high concentrations of electrolytes (particularly iron and sodium) and glucoses. Iron seems likely, because the experience of eating meat so closely parallels that described of blood drinking. If there is an inordinately high number of these receptors, or a genetic variant of this receptor, then that would show a physiological basis for the body's reaction to the taste of blood, as well as the mechanism for creating the psychological association that forms the cybernetic loop that drives the "hunger", even though it (reportedly) doesn't truly fill you as food does.
It would also explain the short term and long term effects perceived from blood drinking. The initial rush would be the body's reaction to the receptors "informing" it that fresh electrolytes were on the way. The body "powering up" in anticipation of the need to metabolize them, and the psychological anticipation of the Pleasure experience. Then, the sluggishness that can occur with the digestion of all foods.And, the long term energy effects of elevating your eletrolyte/iron count in your bloodstream. Just as the amount of blood in your system is self-regulating, like the ability to consume extra fluids after donating, the body probably perceives the new imbalance of electrolytes, and begins to correct that until the natural balance is more closely approximated. Which sets the stage for the entire process to repeat itself.
Perhaps most significantly, the testing for neuroreceptors is something that would never be done as a part of any medical procedure, and so would probably be overlooked completely. And as the self-regulating process of the body would return the bloodstream to a close approximation of its original state, nothing would particularly show up in the blood. Except perhaps very slight cases of the iron and/or electrolyte levels fluctuating, which is to a degree normal. Including mild acute cases of Anemia.
What this leaves us with is a "naturally" (physiologically) occurring process, that results in a what is perceived to be an "unnatural" psychology, through genetic predisposition. Such a "condition", if confirmed scientifically through testing, would show that blood drinkers have no conscious role in the creation of their state. However, it would equally show that they could consciously choose to change this state through the use of cybernetic loop re-association, the same mechanism that solidified it as part their nature.
From everything that I've read and observed, this seems far and away the most comprehensive and plausible scientific theory re: a physiological mechanism of the "Sanguivorous" nature.
