Check out this article, which does a nice job of summarizing what clinical vampirism is all about. Honestly, all I cared about were the nasty-ass case reports. Highlights include the dude who liked to jerk off to the sight of his own blood and had managed to figure out how to cut himself so as to cause blood to spurt into his mouth, and the pregnant woman who was hospitalized on multiple occasions for vomiting large quantities of her own blood, which she ingested via cuts she made in the base of her tongue. I’ve managed to find a couple of other publications of interest.
John Haigh was a fellow from England who in the late 1940s killed at least six people, drank their blood, and then dissolved their bodies in drums of concentrated sulphuric acid. If you give it enough time, it works. You just have to watch out for incriminating things like dentures and steel plates.
Dude just wanted a break from jail, and figured out that making himself sick by ingesting his own blood was a good way to get to the relative luxury of a hospital room. Based on an accidental viewing of Fear Factor, I suppose that drinking a glass of your own blood wouldn’t be all that bad. You’d just have to chug it and hopefully have something to chase it with.
People may also drink their own blood because Zuul/Big Bird/Jesus/the CIA commands them to.