Doctors used leeches because they believed illness came from having too much blood, or from blood being out of balance with the body’s other fluids. For more than 2,000 years, this idea drove one of the most common treatments in medicine: deliberate bloodletting. Leeches were one of the easiest, least painful ways to do it. What’s surprising is that modern science has shown leech saliva contains dozens of genuinely useful compounds, and leeches are still used in hospitals today.
The Theory Behind Bloodletting
Ancient Greek, Egyptian, Indian, and Arab physicians all shared a core belief that the body contained four essential fluids, or “humors”: blood, phlegm, yellow bile, and black bile. Health meant these fluids were in balance. Sickness meant one of them had built up too much, a condition doctors called “plethora” or “congestion.” The logical fix was to drain the excess.
Blood was considered the most abundant humor and the easiest to remove, so bloodletting became the go-to treatment for almost everything. The Greek physician Galen prescribed it for conditions ranging from simple inflammation and fevers to mental disorders and hemorrhoids. Other physicians used leeches to treat gout, rheumatic pain, skin diseases, hearing loss, dental problems, and urinary complaints. Humoral treatments also included induced vomiting, enemas, and dietary changes, but leeches held a special place because they could be applied precisely to a specific area of the body and worked without the mess or danger of cutting open a vein.
Why Leeches Instead of a Knife
Doctors had two main options for drawing blood: a blade (a technique called phlebotomy) or leeches. Phlebotomy required skill and carried risks of cutting too deep or hitting the wrong vessel. Leeches, by contrast, attached themselves, fed until full, and dropped off. They were easy to use, portable, and relatively painless. A single medicinal leech consumes about 2.5 milliliters of blood per feeding. After the leech detaches, the bite continues to ooze for several hours, releasing roughly another 2.5 milliliters. That prolonged bleeding was considered part of the therapy.
By the early 1800s, leech use had reached industrial scale. France alone imported tens of millions of leeches per year, and the animals became a fixture of the European medical economy. Doctors applied them to the temples for headaches, behind the ears for fevers, and over the abdomen for digestive complaints. The leech was so central to medicine that the word “leech” itself became slang for doctor in English.
What Leech Saliva Actually Contains
When a leech bites, it injects saliva loaded with biologically active compounds. The most important is hirudin, a powerful blood thinner first identified in the 19th century by the physiologist John Haycraft. Hirudin works differently from standard pharmaceutical blood thinners. It binds directly to the clotting enzyme thrombin in a one-to-one ratio, blocking it without needing any helper molecules. Hospital-grade blood thinners like heparin, by comparison, require a cofactor protein to work.
But hirudin is only one piece. Leech saliva also contains compounds that dissolve existing clots, widen blood vessels, reduce inflammation, fight bacteria, and numb pain at the bite site. Proteins called bdellins and eglin C suppress overactive immune enzymes that damage tissue during inflammation, which is why leech saliva shows potential in conditions like arthritis. Another compound, hyaluronidase, helps all these substances spread through surrounding tissue. In short, leech saliva is a cocktail that keeps blood flowing, reduces swelling, and prevents clotting, all at once.
How Leeches Are Used in Modern Surgery
Leeches never fully disappeared from medicine, and in 2004 the U.S. Food and Drug Administration officially cleared medicinal leeches as a medical device. Their primary use today is in reconstructive and microsurgery, particularly after procedures like finger reattachment, skin flap transfers, and ear or scalp reconstruction.
The problem leeches solve is venous congestion. When surgeons reattach a severed finger or move a flap of tissue, they reconnect arteries (which pump blood in) and veins (which drain blood out). Veins are smaller and harder to reconnect, so sometimes blood flows into the reattached tissue but can’t drain properly. The tissue swells, turns purple, and can die within hours. Leeches relieve that pressure in two ways: they physically remove pooled blood, and their saliva prevents new clots from forming while encouraging blood vessel growth. The anticoagulant effect of hirudin keeps the bite wound oozing for hours after the leech detaches, providing continuous drainage during the critical window while the body rebuilds its own venous connections.
The saliva’s anti-inflammatory compounds also help protect tissue from the damage that occurs when blood flow is interrupted and then restored, a process called reperfusion injury. This combination of mechanical drainage and chemical therapy makes leeches uniquely effective for a very specific surgical problem.
Risks of Leech Therapy
Leeches carry bacteria in their digestive tracts, most notably species of Aeromonas. These bacteria are symbiotic partners that help the leech digest blood, but they can cause infections in patients ranging from minor wound complications to serious bloodstream infections. Immunocompromised patients face the highest risk. For this reason, hospitals routinely prescribe preventive antibiotics before and during leech therapy. Surface disinfection of the leeches alone isn’t enough to eliminate the bacteria living inside them, so the antibiotics target the patient’s body rather than the leech.
Why the Practice Declined and Returned
Leech use dropped sharply in the late 1800s and early 1900s as medicine shifted toward germ theory, antiseptic surgery, and pharmaceutical drugs. The humoral framework that had justified bloodletting for millennia lost its scientific credibility, and doctors increasingly viewed leeching as outdated superstition. For much of the 20th century, leeches were a punchline about medieval ignorance.
Their comeback started when reconstructive surgeons in the 1980s rediscovered what ancient physicians had stumbled onto by accident: leech saliva genuinely does reduce swelling, prevent clotting, and promote healing. The difference is that modern doctors use leeches for a precise, evidence-based purpose rather than as a cure-all. The old physicians were wrong about why leeches worked, but they weren’t entirely wrong that leeches did something useful. The biology of the leech turned out to be more sophisticated than the theory that put it on the patient.