These people get relief from stress through either drinking their own blood or the blood of animals or other people. It is more common for women thus afflicted to drink their own blood and for men to take blood from others. Many survivors of abuse report incredible pressure to cut themselves and then drink their blood. Once they do this, they feel great release of tension. This can be a demonic compulsion.
Most cultures have taboos against drinking blood.
The psychiatric literature is full of cases of people who drink others' blood, often to the point of murder. A surprising number of these people perform their vampirism in a religious context of some sort, quite frequently rooted in Catholicism. Such was my case. Satan obviously knows what he is doing. True vampirism is not a joke, nor is it a ghost story.