Many heart patients taking blood thinners appear to be unaware that the dietary supplements they take could put them at serious risk, new research finds.

The study, presented at an American Health Association meeting in Chicago, looked at 100 patients with atrial fibrillation, a common heart rhythm problem.

The researchers, from the Intermountain Medical Center Heart Institute in Salt Lake City, found that 35 of the patients were taking dietary supplements along with warfarin, a commonly prescribed medication that helps prevent blood clots.  

The drug, sometimes sold as Coumadin, has what's called a "narrow therapeutic margin," meaning it requires precise dosages. Too much of the drug and the blood becomes thin enough to raise the risk of bleeding; too little and the risk of blood clots and strokes rise.

More than half of the 35 patients in the study were unaware that dietary or herbal supplements could interfere with their warfarin. In fact, of the 100 most-used supplements, 69 per cent interfere with warfarin's effectiveness.

The most commonly used supplements among the patients were: vitamin supplements; glucosamine/chondroitin; fish oil and coenzyme Q10. All can interfere with warfarin.

Warfarin and herbal and dietary supplements can "compete" in the liver, explains Dr. T. Jared Bunch, a study author and heart rhythm specialist. This competition changes the way the blood thinner works and either intensifies the active ingredients or reduces their effectiveness.

The study found those who took supplements reported higher rates of unexplained bleeding compared to those not taking them (23 per cent vs. 17 per cent), and a greater need for blood transfusions (14 per cent vs. 10 per cent).

Patients who reported taking supplements were also more likely to skip their warfarin (34 per cent to 17 per cent) or take extra doses when it was missed.

"We have also learned that – for whatever reason – patients don't want to tell their doctors that they are taking herbal and dietary supplements," Bunch said.

Bunch called the findings alarming and said the study shows that doctors need to do a better job of teaching patients about the dangers of mixing warfarin with these products.

"Physicians must be active in asking about supplement use and not place responsibility on patients. We need to tell our patients that it's acceptable to use herbal and drug supplements, but important for them to tell us so that we can educate them about the benefits, dangers, and potential interactions with their other medications."

According to Health Canada, many supplements can interfere with warfarin. Ginseng, for example, reduces its effects, while ginkgo biloba may increase its effects. Common over-the-counter drug products can also interact with warfarin. They include: acetaminophen; ASA (often sold as aspirin); and acid reducers, such as ranitidine.

Even certain foods high in vitamin K can decrease the effects of warfarin, such as liver, broccoli, brussels sprouts. Patients taking warfarin are warned to avoid sudden changes in their daily intake of these foods.