How anticoagulants work

Anticoagulants are medicines that act on coagulation, the process your blood uses to form a clot. A clot doesn't appear all at once. It's the last step of a cascade, a sequence of steps that works a lot like a factory assembly line. At the end of the line is thrombin, an enzyme, meaning a protein that makes a chemical reaction possible. Thrombin takes fibrinogen, the raw material, and turns it into fibrin, which weaves together like the threads of a fabric to form a stable clot.

Anticoagulants interrupt that line at different points. Some directly block an enzyme: they stop thrombin, or they stop factor Xa, the part of the cascade that makes thrombin. It's like switching off one station in the factory. Others work indirectly. Some attach to antithrombin, the body's natural brake on clotting. Others keep the liver from making the vitamin K-dependent factors, parts of the cascade that need that vitamin. It's as if the assembly line stopped getting its parts delivered.

That's why they prevent clots from forming. But an anticoagulant doesn't break up a clot that's already there; it doesn't work like a pair of scissors cutting through finished fabric. What it can do is keep that clot from growing any bigger.