Despite many years of mockery, leeches are actually making a comeback in the medical world.

Dr. Eric Lis is a physician, gamer, and author of the Skirmisher Publishing LLC sourcebook, Insults & Injuries.

Despite many years of mockery, leeches are actually making a comeback in the medical world.

Medieval healers had many genuine skills and undoubtedly saved countless lives with their arts, but as near as we can tell today, quite a lot of what they did was, at best, not helpful and, at worst, outright harmful. We shouldn't judge them too harshly for this; they didn't have the benefit of the philosophy of the scientific method and the randomized controlled trial, and in any event some of what physicians believe in today will have been debunked a hundred years from now. Nobody's quite sure how or when leeches became an image classically associated with medieval medicine in general and its inadequacies and quackeries in particular, but for many years, the leech was essentially the symbol of Western medicine, so much so that the English word for the animal comes from an Old High German word for "doctor" and not the other way around. With the rise of modern medicine, it rapidly dawned on healers that applying leeches had some very real and negative side effects but probably had minimal benefit (aside from the placebo effect, which should by no means be discounted), and their use wasn't merely abandoned but grew to become little more than a humorous cliché.

So, all of this to say, it ought to come as a surprise to modern folk that leeches are once again finding a place in hospitals, albeit for a much more narrow list of conditions than in the olden days. Just as we've learned that you can't just give an antibiotic to every person with a cough, it seems that "hirudotherapy" can help with a handful of condition, mostly relating to surgeries that nobody was doing back in the days when leeches were really popular.

Leech saliva contains a number of potent chemicals, including the anticoagulant hiruden (take a guess where the name comes from), an anesthetic, a vasodilator to improve local blood flow, and an enzyme called hyaluronidase that makes it easier for fluid to pass through body tissue. After a leech is removed, the powerful anticoagulant (one of the most powerful known, in fact) results in a wound that continues to slowly ooze blood for hours.

"Medicinal leeches," meaning a variety of leech species which are felt to be particularly safe and efficacious medically, are actually an approved "medical device" in the United States, the UK, and Canada, and they mainly see use in patients post-surgery (in fact, 2014 marks ten years since the FDA approved their use). A couple of years ago, a big team published a review article on over 270 cases of patients who had been treated with leeches, and while a series of case reviews doesn't give us the same data as an experimental study would, the paper does present some useful knowledge about the mechanism by which leeches affect the body. In modern medicine, leeches are used in a number of ways. Plastic surgeons use them because as they draw blood into their bodies, general blood flow to the affected body part improves, and this seems to improve the survival of skin grafts and skin flaps. Vascular surgeons use them to relieve venous congestion; when blood can't flow out of a tissue, the excess blood can cause ulcers from pressure or start to clot in the veins, and a leech can relieve the pressure in the congested tissue. Leech-induced blood flow seems to improve the outcome, under certain circumstances, of replantation of severed ears, fingers, and other organs (leech therapy is considered particularly useful to improve the success rate of penis reattachment; I'll give the men in the audience a moment to think about all the implications of that). The authors note a lot of inconsistency in how the leeches were used... published case studies differed widely in how many leeches were used and how often they got changed. In India and other less hirudophobic countries, leeches are used in such diverse illnesses as diabetes, heart disease, cancer, infections, and arthritis, but the evidence for such uses tends to be very poor.

The authors also discuss some of the side effects of hirudotherapy, which is perhaps the most useful part for us as fantasy enthusiasts. Most obviously, someone with leeches on their body for 24 hours will experience a certain amount of blood loss, and in the cases reviewed, about half of all people needed blood transfusions. Blood transfusions probably aren't safely available in your  fantasy setting, so this can pose a real health risk to a commoner with a constitution of 10 and a dietary iron deficiency. About one in five people experienced some other complication of leech therapy, notably infections (I honestly don't know how one goes about sterilizing a leech prior to use, but the majority of patients studied received preventative antibiotics), scars from the bites, kidney failure (due to dehydration and blood loss). Only one case in over 270 reported that the patient complained of pain. Many patients have problems with mild allergic reactions to leech saliva but I couldn't find any reports of deadly allergic reactions.

Interestingly, psychosis is apparently a complication of leech therapy. I was curious about this, so I did some further reading. Sadly, since data on leech therapy is sparse to begin with, I found very few papers on the topic. The authors of one paper make the delightful observation that "many patients are apprehensive about this type of therapy" and noted that little data has been collected on what percentage of patients actually refuse or for psychological reasons can't complete a course of leech therapy. They discuss that the general consensus among users of leech therapy is to give a patient option to see the leech before therapy and the option to have a visual barrier put up during therapy, so at least they don't have to watch. Since leeches are often kept at the bedside in between applications, the authors also suggest putting them in an opaque container. They make no mention of the possibility of people actually becoming psychotic due to hirudotherapy, but another paper notes that many patients requiring leech therapy in modern times need it because they've had a very traumatic accident, and they can spend over a week in the intensive care unit it. As any intensivist will tell you, simply being in the ICU is a risk factor for delirium and psychosis, and this probably isn't an issue for the characters in your games.