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Heart Disease Health Center

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Warfarin, Dietary Supplements a Risky Combo

Study Shows Many Patients Using Anticlotting Drug Don’t Tell Doctor About Dietary Supplement Use
WebMD Health News
Reviewed by Laura J. Martin, MD

Nov. 16, 2010 (Chicago) -- If you're using the anticlotting drug warfarin, tell your doctor about any herbal or dietary supplements you may be taking.

That's the strong advice of researchers who say that nine of the 10 top-selling supplements can change the effectiveness of warfarin, potentially causing a dangerous bleed, a deadly blood clot, or even a stroke.

In a survey, nearly three-fourths of 100 people on warfarin reported they used over-the-counter multivitamins or other supplements, yet supplement use was not documented on the medical charts of nearly 70% of them, says Jennifer Strohecker, PharmD, a clinical pharmacist at Intermountain Healthcare in Salt Lake City.

Previous research has shown that fewer than one in three people tell their doctor about dietary supplement use, she tells WebMD.

The new findings were presented here at the American Heart Association’s annual meeting.

Stroke and Bleeding Risk

People taking warfarin, which is also sold under the brand name Coumadin, require regular blood tests to determine the INR, a measure of the clotting tendency of blood. If INR levels go too high, you're at risk of suffering a dangerous bleed; too low, and you’re at risk for a deadly blood clot or stroke.

"If you add in another medication or supplement, you can alter the INR," says American Heart Association spokesperson Nieca Goldberg, MD, of New York University's Cardiac and Vascular Institute.

Three of the top-selling supplements lower the INR and raise stroke risk when combined with warfarin: soy, multiherbal supplements, and coenzyme Q10, Strohecker says.

The risk of bleeding is increased if you take the anticlotting medication with any of six best-sellers: cranberry, melatonin, antioxidants, fish oil, and the joint supplements glucosamine and chondroitin, she says.

Many dangerous warfarin-related drug interactions could be avoided with better doctor-patient communication, Strohecker says. "One of the biggest problems we found is that while doctors routinely ask about prescription drug use, they don't ask about use of supplements. And patients generally do not view supplements as drugs."

Overall, 27 of the 40 top-selling dietary supplements -- including minerals, vitamins and herbs -- have the potential to interact dangerously with warfarin, according to the researchers.

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