Q. Why do some people go crazy when they drink the blood of drained vampires? Why doesn't someone get high when taking blood right from the vampiric source?
A. The basic answer is: everything is variable when it comes to vampire blood being ingested by humans. There's no scientific formula that says an ounce of vamp blood will make everyone go nuts. 'V' is like any other street drug in that respect. The variables include the age of the blood (how long it has been out of the vampire), the age of the vampire (the older, the more potent), as well as the mental condition and previous drug use of the imbiber. Blood taken straight from the vampire doesn't necessarily make the drinker high, unless s/he is already an addict.
Q. Can vampires gain sustenance off of something other than Human blood, or synthetic?
A. In short, no. A properly turned vampire will not be satiated by anything other than Human blood/synthetic. A vampire that is poorly turned though, can often gain sustenance from any mammalian creature that creates its own blood. (In the Southern Vampire Mysteries, Bubba quite enjoys eating cats.)
Q. Can vampires eat/drink human food?
A. Food is a tricky, tricky thing. Vampires can, and do, enjoy and savour the flavors of various food (with the exception of garlic which they can't stand) in the blood that they're drinking — it's why more than a few still feed from humans once in a while, even though synthetic may be readily available. They cannot, however, eat human food. Canonically, they don't even really enjoy if their human companions smell like certain foods. (There is an entire portion of a chapter where Sookie is brushing her teeth after eating because Vampire Bill doesn't like to watch her eat and doesn't like her to taste like food.) They do not really drink anything beyond blood either — no coffee, no alcohol, no water — though they can mix a synthetic blood into their beverage and drink it. They can feign eating/drinking (Bill orders a glass of wine, for instance), but they can't really enjoy the food/beverage. The reasons for not eating or drinking is simple: their intestinal system doesn't really function like a human's, and therefore they cannot process or digest food at all.
