Quote Originally Posted by ProjectEuropa

Can't think of the condition at the moment but there is a condition where one requires the nutrients of blood, mainly iron to function properly. Without it one suffers pain and irritation when in the light, hence the legend of vampires only coming out in the dark. It has been said that Vlad the Impaler had this condition. It was sometime ago I heard this so maybe someone more in the know can enlighten me as to whether this is correct.
You're thinking of Porphyria - suspected to be behind the "madness of king george". Among other things, symptoms include purple wee, mental disturbances, receeding lips and gumlines (making the teeth look longer) and sun sensitivity (sufferers burn and blister easily).

The main problem for sufferers is the inability to produce haem, an important part of your blood. Unfortunately for the theory that it is the root of the vampire legends, the blood parts that porphyria sufferers need are broken down by the digestive system - so drinking blood doesn't help (although injections of blood products can).

It doesn't sound like a sexy disease to me. If you really want to turn your piss purple, just eat a lot of beetroot.