With such a high interest in vampires these days — they're all over television, movies, and bookstore shelves — many people may be wondering if humans really can survive as vampires. 

Turns out, sucking blood isn't just fiction fun: In 2011, a 19-year-old Texas man named Lyle Bensley allegedly broke into a woman's apartment and bit her on the neck. This was not bedroom playacting between lovers; Bensley claimed to be a centuries-old vampire who needed blood to stay alive. The woman escaped and called police, and Lyle the would-be vampire was arrested for assault.

But is it safe to drink blood?

In very small amounts (say, a few teaspoons), and if the blood is free from pathogens (such as the many blood-borne diseases), blood might not harm you. Beyond that, watch out. [Circulatory System: Facts, Function & Diseases]

The strange fact is, blood, when drank, is toxic. When confined to places where blood is supposed to be — such as the heart, vessels, and so on — it is essential for life. But when ingested it's a very different story. Of course all toxins have doses, and just as a tiny bit of poison won't necessarily harm you, the more you eat or drink, the greater the danger. [7 Strange Ways that People Act Like Vampires]

Because blood is so rich in iron — and because the body has difficulty excreting excess iron — any animal that consumes blood regularly runs a risk of iron overdose. While iron is necessary for all animals (and indeed most life), in high doses it can be toxic. This condition, called haemochromatosis, can cause a wide variety of diseases and problems, including liver damage, buildup of fluid in the lungs, dehydration, low blood pressure, and nervous disorders.

Even so, blood-feeding in humans seems to be more common than one might expect. John Edgar Browning of Georgia Tech, who has researched the practice, suggests in a BBC article that thousands of people across the United States drink blood for various reasons. Some of these people say the oxygen- and iron-enriched substance gives them energy, and none of the drinkers interviewed by Browning have had any ill effects from their behavior, so perhaps they didn't overdo the blood meals: "No vampire I’ve interviewed has complained of any medical complications as a result of consuming blood," Browning said, according to BBC.

Unlike humans, the bodies of animals that digest blood have adapted specialized digestive mechanisms. According to Katherine Ramsland in her book "The Science of Vampires" (Penguin Putnam, 2002) the vampire bat, "requires an enormous intake of iron, which helps make hemoglobin for carrying oxygen from the lungs to the body tissues. Yet the iron intake is generally higher than what the bat needs, so it has a special process for secreting the excess. When ingested, the blood goes through a tract that's adapted for extracting nutrients. Research on this system suggests that bats have a mucous membrane along the intestinal tract that acts as a barrier to prevent too much iron from getting into their bloodstreams."

You, however, are not a vampire bat. Because humans did not evolve such an iron-extracting mechanism, drinking blood can kill us.

If you're thinking of sampling human blood, make sure there's a doctor handy — for you, not your victim.

Editor's Note: This article was first published on Sept. 2, 2011.

Benjamin Radford is deputy editor of Skeptical Inquirerscience magazine and author of six books including Scientific Paranormal Investigation: How to Solve Unexplained Mysteries.His Web site is www.BenjaminRadford.com.