warfarin
Meaning of warfarin: Food, alcohol and dozens of other drugs shift its effect, which is why the dose is set by blood test rather than by body weight.
Definition of warfarin
Warfarin is an oral anticoagulant that blocks the recycling of vitamin K in the liver. Without recycled vitamin K, the liver cannot finish building clotting factors two, seven, nine and ten, so blood takes longer to clot. It does not thin the blood and it does not dissolve a clot already present; it lowers the chance that a new one forms or an existing one grows.
How does blocking vitamin K work?
Several clotting factors need a final chemical finishing step before they can work, and that step consumes vitamin K. An enzyme in the liver regenerates the vitamin after each use, and warfarin blocks that enzyme, so the recycling stops and the supply runs down. Factors already circulating keep working until they are cleared naturally, which is why the full effect takes several days to arrive and several days to fade after a dose changes. The same vitamin is needed for two natural anticoagulant proteins, and because one of those disappears fastest of all, the first day or two can briefly tip toward clotting rather than away from it. That is the reason an injected anticoagulant is often overlapped at the start when a blood clot is already present.
Why does it need monitoring?
| Influence | Effect |
|---|---|
| Vitamin K in food, mainly green leafy vegetables | Competes with the drug, so a sudden change in intake shifts the result |
| Antibiotics | Reduce vitamin K made by gut bacteria and can also slow warfarin breakdown |
| Other medicines and supplements | Dozens compete for the same liver enzymes in both directions |
| Illness, alcohol, thyroid changes, inherited enzyme differences | All alter how much drug is cleared or how fast factors are used |
Clotting time is measured as the prothrombin time and reported as the INR, a standardized ratio that lets results from different laboratories be compared. The dose is then adjusted to hold the INR in a target range chosen for the condition being treated.
How is it reversed?
Vitamin K restores clotting over hours, and concentrated clotting factor products do it within minutes when bleeding is severe. Newer oral anticoagulants block a single clotting factor directly, need no routine monitoring and interact with far fewer foods and drugs, but warfarin remains in use for situations such as mechanical heart valves where those alternatives have not proved suitable. Bleeding is the main side effect of any drug in this class.
Also known as
Used in a sentence
Warfarin was continued with regular laboratory monitoring to keep the anticoagulant effect within the target range.
Good to know
Warfarin's effect shifts with diet, illness and other medicines, which is why clotting time is measured repeatedly rather than assumed to stay constant on a fixed dose.