What Is Warfarin? - Uses & Side Effects

Instructor: Catherine Konopka

Catherine has taught various college biology courses for 5 years at both 2-year and 4-year institutions. She has a Ph.D. in cell and molecular biology.

Blood clotting is a life-saving process, but it can lead to life-threatening strokes if not controlled. In this lesson you will learn about the small molecule pharmaceutical, warfarin, that reduces the rate and occurrence of blood clotting and helps prevent strokes.

Bleeding Versus Blood Clotting

Getting a small cut isn't a big deal for most of us, but for those of us with defects in blood clotting, small cuts can be deadly! If your blood doesn't clot enough, you can bleed too much and for too long. On the other hand, if your blood clots too much or at the wrong time, a thrombus (a blood clot on the inside wall of an uninjured blood vessel), can form.

Our bodies generally work to prevent both of these situations. They regulate the clotting process so that the right amount of clotting happens at the appropriate time. There are proteins in your blood that, when activated, can cause your blood to coagulate, or to clot and become gel-like. There are also proteins in your blood that are anticoagulants, which prevent clotting. By altering the activation of proteins on one side or the other, blood clotting can be controlled.

Think of your blood like a seesaw at a playground. As long as there are equal kids (or proteins) weighing about the same amount on either side, the seesaw is balanced -- not too much clotting to cause a thrombus and not too little to cause bleeding. If you take one kid off of the 'coagulation' side, the seesaw will tip toward the 'no clotting' side. This is essentially what happens with the use of anticoagulant pharmaceuticals like warfarin and heparin. These small molecules interfere with the proteins that help the blood clot. The most common commercial name of warfarin is Coumadin™.

How Does Warfarin Work?

In order to understand how warfarin works, we first need to consider a simplified version of how blood clots are formed. The inside wall of a healthy blood vessel is lined with cells that act as a barrier between the circulating blood and surrounding tissue. When the blood vessel wall is damaged, the inner cell layer is disturbed and all those proteins in the blood are now exposed to the surrounding tissue.

One of the 'pro-coagulating' proteins in the blood is activated upon exposure to tissue. This protein then activates other proteins, which activate other proteins, and so forth. Eventually a protein called thrombin is activated, which in turn converts a different protein called fibrinogen to fibrin. Fibrin cross-links to other fibrin molecules to form a dense meshwork to repair the blood vessel. This meshwork is the actual clot. Picture a scab -- a scab is just a bunch of cross-linked fibrin strands with trapped blood components.

To get a better handle on all of this 'activation,' think of the activated proteins like superheroes. The unactivated forms are their alter egos - laying low until they get the call to action. Let's suppose that Clark Kent is the first protein in the clotting cascade. He learns of a break in a nearby dam and turns -- or 'activates' -- into Superman. Instead of just fixing the dam himself, Superman tells Peter Parker to get his Spiderman outfit on, who tells Diana Prince to slip into her Wonder Woman garb, and so forth. The last superhero activated…let's go with the Hulk…throws himself into the hole to plug up the dam.

Now that we have a handle on blood clotting, where does warfarin come in? Warfarin prevents the first few alter egos from transforming -- or activating -- into superheroes. Several of the early activation steps in the pathway require vitamin K. Warfarin limits the amount of usable vitamin K. Without a useful form of vitamin K, the early activation steps in clotting won't happen, which means thrombin won't get activated, fibrinogen won't change into fibrin, and the clot won't form.

Warfarin blocks the early steps in the blood clotting pathway.
Warfarin blocks early steps in the blood clotting pathway

Uses and Side Effects

When clotting occurs too frequently or in the absence of vessel injury, a thrombus forms. The thrombus prevents the normal flow of blood through a blood vessel. If a thrombus forms near a vital organ, the blood supply to that organ could be diminished. If this happens for a long enough period of time, an organ can become damaged or nonfunctional.

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