Warfarin, Dietary Supplements a Risky Combo

Study Shows Many Patients Using Anticlotting Drug Don’t Tell Doctor About Dietary Supplement Use

Medically Reviewed by Laura J. Martin, MD on November 16, 2010

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.

Dietary Supplement Survey

For the current study, Strohecker and colleagues surveyed 100 patients being treated with warfarin. Among the findings:

  • Only about one-third of the patients said their doctor asked them about supplement use, but 71% who were asked told their doctor about their use.
  • Nearly half do not view supplements as drugs and two-thirds do not consult with their pharmacist or doctor before taking a supplement.
  • Almost one-third said they prefer herbal or other dietary supplements over traditional medications.

Goldberg tells WebMD that since dietary supplements are sold over the counter in retail stores, most people just assume they are safe.

In fact, supplements are not subject to the same regulation as drugs, so their strength and purity can't be assured, she says.

This study was presented at a medical conference. The findings should be considered preliminary as they have not yet undergone the "peer review" process, in which outside experts scrutinize the data prior to publication in a medical journal.

Show Sources


American Heart Association's Scientific Sessions 2010, Chicago, Nov. 13-17, 2010.

Jennifer Strohecker, PharmD, clinical pharmacist, Intermountain Healthcare, Salt Lake City.

Nieca Goldberg, MD, Cardiac and Vascular Institute, New York University.

© 2010 WebMD, LLC. All rights reserved. View privacy policy and trust info