One of the strangest prescription drugs that you can be given today isn’t a drug at all— say hello to Hirudo Medicianalis, the medicinal leech. Taking a cue from medieval medicine, the FDA has approved leeches as a legitimate medical device.

The decision seems questionable and downright icky when you first see these little medical monsters. Leeches are a type of water-based worm with a suction cup mouth and a battalion of tiny teeth. Their skin is slick and patterned with muted geometric stripes. This pattern camouflages the leech in muddy water, where it lays in wait to engorge itself on the blood of dogs, cattle, and of course, humans.

Employing this slimy parasite in a sterile medical procedure sounds like an idea from a horror movie. Yet when it comes to moving and directing blood, the parasite is nature’s expert. Its saliva contains antibiotics, anaesthetics, and anticoagulants to prevent blood clotting. The leech itself is a suction machine, made to siphon a large quantity of blood. Taken together, these qualities give the leech the power to remove clotted, stopped blood from a wound or an obstructed vein. Modern doctors find that hirudotherapy, or the use of medicinal leeches, is one of the best ways to prevent the onset or advance of gangrene.

But modern hirudotherapy is just starting to hit its stride. Doctors have found that the leech’s saliva is a valuable medicinal discovery on its own. Leech saliva can be harvested without hurting the leeches, and thus collected to treat internal medical problems as well. It is being examined as a possible preventative measure for clot-related diseases like heart attacks, thrombosis, and strokes.

Thanks to their saliva and other medicinal properties, leeches may finally be able to repay us humans for the blood they’ve been stealing over the ages.