"First of all, it absolutely, 100 percent should be discouraged by everybody. The safety issues are gargantuan," said Keith-Thomas Ayoob, an associate professor at the Albert Einstein College of Medicine in New York City.
There's an enormous risk of contracting blood-borne diseases, he said.
Underscoring the dearth of evidence for Renfield's syndrome, author Ramsland said she knows the clinical psychologist who made up the term as a joke.
But in very rare cases, she said, people suffer from clinical vampirism, which is the psychotic delusion that you need blood to survive. Convinced that they need to drink blood, she said, some people cut their own arteries or have killed loved ones.
This condition, she emphasized, is very different from forming a persona around a vampire and participating in a subculture that celebrates it.
Merticus, a 30-year-old "hybrid" vampire who is also a member of the invitation-only service group Voices of the Vampire Community, said he feeds on both energy and blood, adding that his primary feeding method is pranic and tantric-sexual, meaning that he feeds on sexual energy and arousal.
The Atlanta-based antique dealer chose not to disclose his legal name to ABCNews.com.
Although the precise reason for craving blood is unclear, vampires "cannot adequately sustain their own physical, mental or spiritual well-being without the taking of blood or vital life force energy from other sources, often human," he told ABCNews.com.
Sanguine vampires feed by drinking human or animal blood but vary in their experience of blood-hunger, he said. They typically consume an ounce or less of blood at a time, usually no more than once a week. When blood is from human sources, he said, it is consensual and facilitated through verbal or written contracts between vampires and donors.
"The vampire-donor relationship is one of mutual respect and gratification," he said.
"We make every effort to educate ourselves on safe feeding methods, basic anatomy and physiology, first aid, sterilization, disease prevention, and safer sex practices," he told ABCNews.com. "We thoroughly screen donors for both physical and mental health conditions and concerns, as well as advocate [for] frequent updates in testing."
They don't feed from those who are knowingly infected with HIV, hepatitis or other blood-borne diseases, he said. And they avoid those whose physical conditions place them at risk of harm by sanguine or pranic feeding.
E. Mark Stern, an independent psychotherapist, professor emeritus at Iona College in New York and a widely published author on psychotherapy, has not studied the vampire subculture specifically but has dealt with a number of people who have claimed vampire tendencies.
He recognizes that there are certainly radical manifestations of the phenomenon, noting that some cults have exploited the blood theme to perpetrate fatal crimes.
But aside from these examples, he said, using blood as a way to bond people to a community is not entirely beyond the mainstream.
For example, taking communion in the Catholic tradition means, metaphorically, receiving the flesh and blood of Christ. Some Orthodox Jews practice a controversial circumcision ritual in which the rabbi performing the circumcision sucks some of the blood from the child's wound to clean it.
But the blood bond also exists outside religion.