Updated February 29, 2016

If you've had blood clots in the past, there's a good chance that you currently have a prescription for warfarin (brand name Coumadin) or a similar anticoagulant. Though they are often referred to as blood thinners, these medications don't actually thin the blood. Instead they block vitamin K, also known as "the clotting vitamin." The liver contains several proteins that contribute to clotting, and they all depend on a steady supply of vitamin K. By keeping this nutrient from doing its job, blood thinners can lower the risk of dangerous clots that can cause heart attack or stroke.

Barry Kirschner, 38, of Virginia Beach, Va., has taken Coumadin every day since having a heart valve replaced in February 2007. For him it has been an easy adjustment. "Coumadin gets a bad rap," he says. "But as long as you're not binging on high vitamin K foods like spinach, it's not a big deal."

It takes a real balancing act to prevent dangerous clots. If your drug isn't working well enough, you could suffer a stroke or heart attack. But if it's working too well, your blood won't stick together even when it's supposed to, and you could start bleeding internally. Just as patients with diabetes regularly check their blood sugar, patients taking Coumadin or similar drugs can use a home monitor that measures the clotting time of a drop of blood. If the drop doesn't clot too quickly or too slowly, patients can be confident that they've reached the right balance.

At first Shannon Schroeder, of Poulsbo, Wash., had trouble reaching the safety zone. She took her medication as prescribed, but blood tests showed that her blood still clotted too quickly. After talking with her doctor, she realized that her longtime habit of eating leafy greens was putting her at risk. These usually heart-healthy vegetables are loaded with vitamin K, the very thing that Coumadin is supposed to block, and therefore a patient's intake needs to be calibrated with the dosage of the drug. Schroeder's doctor adjusted her dose, she adjusted her diet, and now the 37-year-old is right where she needs to be, safe and clot free.

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