new research review published in the Journal of the American College of Cardiology says there are 10 supplements that are particularly risky for people to take if they also take drugs for high blood pressure, high cholesterol, and other heart problems.

These herbal supplements have the potential to reduce the effectiveness of heart drugs and/or cause dangerous side effects.

“Natural does not mean safe," says the lead study author, Graziano Onder, M.D., Ph.D., an associate professor of medicine at the Universita Cattolica del Sacro Cuore in Rome. Onder and his colleagues summarized the evidence on the effectiveness and side effects of Asian ginseng, astragalus, flaxseed oil, garlic, ginkgo, grapeseed, green tea, hawthorn, milk thistle, and soy.

In particular, green tea, garlic, ginkgo, ginseng, and hawthorn have the potential to reduce the efficacy of certain heart medications or increase their side effects. Onder says that people taking aspirin or blood thinners such as warfarin should be especially wary, because supplements paired with those medications can increase the risk of internal bleeding. And ginkgo has been associated with potentially severe side effects including, in rare cases, brain hemorrhaging.

The researchers also concluded that there's no clear evidence that supports the use of any of these 10 supplements to promote heart health. Flaxseed oil, garlic, grapeseed, green tea, hawthorn, milk thistle, and soy all have "limited evidence of benefits," the researchers said, meaning they might help but more research is needed. The other herbs were found to be completely ineffective or there was conflicting evidence about their potential benefit.

Taking these supplements with medications is concern especially when patients don't tell their doctors. According to a nationally representative survey of nearly 2,000 Americans by the Consumer Reports National Research Center, more than half of people who take supplements don’t talk to their primary care doctor about it. 

The Risks of Herbal Medicines

Doctors may not ask you specifically about herbal supplements you may be taking, but you should speak up anyway. "Always report and discuss with your doctor the use of herbal medications,” Onder says. Better yet, talk with your doctor before you start taking one.

Duffy MacKay, N.D., the senior vice president of scientific and regulatory affairs at the Center for Responsible Nutrition, a trade association for the supplements industry, characterizes the authors' conclusions about the lack of proven benefits as "not a fair portrayal of the evidence." But he agrees with a key message of the paper. "Patients should always discuss the benefits and side effects of herbal medications with their physicians," he says. "That's very reasonable and rational."

Consumers should be wary of all supplements, however, even those that seem promising and relatively safe, cautions Pieter Cohen, M.D., an assistant professor of medicine at the Harvard Medical School and an internist at the Cambridge Health Alliance.

As Consumer Reports has previously reported, loose regulation means that the quality and purity of the supplements you buy can vary widely, and it's almost impossible to be sure you're getting what you think you are. "Consumers overwhelmingly assume supplements are safe or they wouldn't be on the shelf," says Chuck Bell, programs director for Consumers Union, the policy and mobilization arm of Consumer Reports. "They generally haven't thought of pitfalls such as drug interactions."

In addition, supplements may be contaminated with pesticides, heavy metals, bacteria, fungi, or even active pharmaceuticals, Cohen noted in a 2010 paper published in Cardiovascular Therapeutics.

If you're interested in trying out something like garlic, for example, Cohen suggests incorporating the actual food into your diet rather than rolling the dice with a supplement.