What can cause porphyria victims to crave blood?

Old legend: There's an idea, probably ill-founded, that congenital erythropoietic porphyria, which produces hairiness, a mutilated face, red teeth, and pain on going into sunlight is the origin of vampire / werewolf tales. The tale that these people crave blood (this isn't a classic feature) may have been added as the story grew, though heme is one factor that suppresses some forms of the illness.