An anticoagulant was a substance that delayed or prevented the clotting of blood.
In 2153, Doctor Phlox located a marsupial on a class L planet whose saliva was a natural anticoagulant. (ENT: "Bounty")
Jem'Hadar disruptors incorporated an anticoagulant into the energy stream; this prevented wounds inflicted by the weapon from healing. (DS9: "The Ship", "Change of Heart")